a Middle English merger of Old English line "cable, rope; series, row, row of letters; rule, direction," and Old French ligne "guideline, cord, string; lineage, descent" (12c.), both from Latin linea "linen thread, string, plumb-line," also "a mark, bound, limit, goal; line of descent," short for linea restis "linen cord," and similar phrases, from fem. of lineus (adj.) "of linen," from linum "linen" (see linen).
The earliest sense in Middle English was "cord used by builders for taking measurements;" extended late 14c. to "a thread-like mark" (from sense "cord used by builders for making things level," mid-14c.), also "track, course, direction." Meaning "limit, boundary" (of a county, etc.) is from 1590s. The mathematical sense of "length without breadth" is from 1550s. From 1530s as "a crease of the face or palm of the hand." From 1580s as "the equator."
Sense of "things or people arranged in a straight line" is from 1550s. Now considered American English, where British English uses queue (n.), but the sense appears earliest in English writers. Sense of "chronologically continuous series of persons" (a line of kings, etc.) is from late 14c.
Meaning "one's occupation, branch of business" is from 1630s, according to OED probably from misunderstood KJV translation of II Corinthians x.16, "And not to boast in another mans line of things made ready to our hand," where line translates Greek kanon which probably meant "boundary, limit;" the phrase "in another man's line" being parenthetical.
Commercial meaning "class of goods in stock" is from 1930, so called from being goods received by the merchant on a line in the specific sense "order given to an agent" for particular goods (1834). Insurance underwriting sense is from 1899. Line of credit is from 1958.
Meaning "series of public conveyances" (coaches, later ships) is from 1786; meaning "continuous part of a railroad" is from 1825. Meaning "telegraph wire between stations" is from 1847 (later "telephone wire"). Meaning "cord bearing hooks used in fishing" is from c. 1300. Meaning "policy or set of policies of a political faction" is 1892, American English, from notion of a procession of followers; this is the sense in the political party line, and, deteriorated, it is the slang line that means "glib and plausible talk meant to deceive."
In British army, the Line (1802) is the regular, numbered troops, as distinguished from guards, auxiliaries, militia, etc. In the Navy (1704) it refers to the battle line (the sense in ship of the line, which is attested from 1706).
Dutch lijn, Old High German lina, German Leine, Old Norse lina "a cord, rope," are likewise from Latin. Spanish and Italian have the word in the learned form linea. In continental measurements, a subdivision of an inch (one-tenth or one-twelfth in England), attested in English from 1660s but never common. Also see lines.
To get a line on "acquire information about" is from 1903. To lay it on the line is from 1929 as "to pay money;" by 1954 as "speak plainly." End of the line "as far as one can go" is from 1948. One's line of work, meaning "pursuit, interest" is from 1957, earlier line of country (1861). Line-drawing is from 1891. A line-storm (1850) is a type supposed to happen in the 10 days or two weeks around the times the sun crosses the equator.
"a featherless plantigrade biped mammal of the genus Homo" [Century Dictionary], Old English man, mann "human being, person (male or female); brave man, hero;" also "servant, vassal, adult male considered as under the control of another person," from Proto-Germanic *mann- (source also of Old Saxon, Swedish, Dutch, Old High German man, Old Frisian mon, German Mann, Old Norse maðr, Danish mand, Gothic manna "man"), from PIE root *man- (1) "man." For the plural, see men.
Sometimes connected to root *men- (1) "to think," which would make the ground sense of man "one who has intelligence," but not all linguists accept this. Liberman, for instance, writes, "Most probably man 'human being' is a secularized divine name" from Mannus [Tacitus, "Germania," chap. 2], "believed to be the progenitor of the human race."
Specific sense of "adult male of the human race" (distinguished from a woman or boy) is by late Old English (c. 1000); Old English used wer and wif to distinguish the sexes, but wer began to disappear late 13c. and was replaced by man. Universal sense of the word remains in mankind and manslaughter. Similarly, Latin had homo "human being" and vir "adult male human being," but they merged in Vulgar Latin, with homo extended to both senses. A like evolution took place in Slavic languages, and in some of them the word has narrowed to mean "husband." PIE had two other "man" roots: *uiHro "freeman" (source of Sanskrit vira-, Lithuanian vyras, Latin vir, Old Irish fer, Gothic wair; see *wi-ro-) and *hner "man," a title more of honor than *uiHro (source of Sanskrit nar-, Armenian ayr, Welsh ner, Greek anēr; see *ner- (2)).
Man also was in Old English as an indefinite pronoun, "one, people, they." It was used generically for "the human race, mankind" by c. 1200. As a word of familiar address, originally often implying impatience, c.1400; hence probably its use as an interjection of surprise or emphasis, since Middle English but especially popular from early 20c.
As "a woman's lover," by mid-14c. As "adult male possessing manly qualities in an eminent degree," from 14c. Man's man, one whose qualities are appreciated by other men, is by 1873. Colloquial use of the Man for "the boss" is by 1918. To be man or mouse "be brave or be timid" is from 1540s. Meaning "piece with which a game (especially chess) is played" is from c. 1400.
Man-about-town "man of the leisure class who frequents clubs, theaters, and other social resorts" is from 1734. Man of the world is from mid-14c. as "secular man, layman;" by early 15c. as "man experienced in the ways of the world, one able to take things in stride." To do something as one man "unanimously" is from late 14c.
So I am as he that seythe, 'Come hyddr John, my man.' 
MANTRAP, a woman's commodity. [Grose, "Dictionary of the Vulgar Tongue," London, 1785]
At the kinges court, my brother, Ech man for himself. [Chaucer, "Knight's Tale," c. 1386]
late Old English mægester "a man having control or authority over a place; a teacher or tutor of children," from Latin magister (n.) "chief, head, director, teacher" (source of Old French maistre, French maître, Spanish and Italian maestro, Portuguese mestre, Dutch meester, German Meister), contrastive adjective ("he who is greater") from magis (adv.) "more," from PIE *mag-yos-, comparative of root *meg- "great." The form was influenced in Middle English by Old French cognate maistre.
From late 12c. as "man eminently or perfectly skilled in something," also "one who is chief teacher of another (in religion, philosophy, etc.), religious instructor, spiritual guide." Sense of "master workman or craftsman, workman who is qualified to teach apprentices and carry on a trade on his own account" is from c. 1300. The meaning "one charged with the care, direction, oversight, and control of some office, business, etc." is from mid-13c.; specifically as "official custodian of certain animals kept for sport" early 15c. (maister of þe herte houndes; the phrase master of the hounds is attested by 1708). As a title of the head or presiding officer of an institution, late 14c.; as "captain of a merchant vessel" early 14c.
In the broadest sense, "one who has power to control, use, or dispose (of something or some quality) at will," from mid-14c. Also from mid-14c. as "one who employs another or others in his service" (in which sense the correlative word was servant, man, or apprentice); also "owner of a living creature" (a dog, a horse, also, in ancient contexts a slave); paired with slave in the legal language of the American colonies by 1705 in Virginia.
In academic sense "one who has received a specific degree" (translating Medieval Latin magister) it is attested from mid-13c., originally "one who has received a degree conveying authority to teach in the universities;" master's degree, originally a degree giving one authority to teach in a university, is from late 14c.
Also used in Middle English of dominant women. From 1530s as "male head of a household." As a title or term of respect or rank, mid-14c. As a title prefixed to the name of a young gentleman or boy of the better class not old enough to be called Mr., short for young master (late 16c.). Sense of "chess player of the highest class at national or international level" is by 1894. Meaning "original of a recording" is by 1904.
As an adjective from late 12c. Master-key, one that will open ("master") a number of locks so differently constructed that the key proper to each will open none of the others" is from 1570s. Master race "race of people considered to be pre-eminent in greatness or power" (typical in reference to Nazi theories of the Aryan race, perhaps based on German Herrenvolk) is by 1935. From 1530 as "artist of distinguished skill;" old masters is attested by 1733.
Master bedroom, "bedroom designed for the use of the owner of the property," as opposed to bedrooms for children or guests, is by 1919 in U.S. home-builders publications (e.g. Building Age, April 1919). It seems to be based on the English master's bedroom (by 1903) "bedroom of a headmaster or other master at an English boarding school or other similar institution."
The top floor was treated much the same as the two lower ones. Here the closet was made just a bit larger so as to allow for a bathtub, thus pushing the partition forward, making the front room less deep than the rooms below, yet paradoxically larger, because it takes in the whole front of the house. This is what is known in English advertisements as the "Master's bedroom." [The House Beautiful, June 1921]
late 14c., "full of joy, merry; light-hearted, carefree;" also "wanton, lewd, lascivious" (late 12c. as a surname, Philippus de Gay), from Old French gai "joyful, happy; pleasant, agreeably charming; forward, pert; light-colored" (12c.; compare Old Spanish gayo, Portuguese gaio, Italian gajo, probably French loan-words). Ultimate origin disputed; perhaps from Frankish *gahi (related to Old High German wahi "pretty"), though not all etymologists accept this.
Meaning "stately and beautiful; splendid and showily dressed" is from early 14c. Of things, "sumptuous, showy, rich, ornate," mid-14c. of colors, etc., "shining, glittering, gleaming, bright, vivid," late 14c.; of persons, "dressed up, decked out in finery," late 14c.
In the English of Yorkshire and Scotland formerly it could mean "moderately, rather, considerable" (1796; compare sense development in pretty (adj.)).
The word gay by the 1890s had an overall tinge of promiscuity -- a gay house was a brothel. The suggestion of immorality in the word can be traced back at least to the 1630s, if not to Chaucer:
But in oure bed he was so fressh and gay
Whan that he wolde han my bele chose.
Slang meaning "homosexual" (adj.) begins to appear in psychological writing late 1940s, evidently picked up from gay slang and not always easily distinguished from the older sense:
After discharge A.Z. lived for some time at home. He was not happy at the farm and went to a Western city where he associated with a homosexual crowd, being "gay," and wearing female clothes and makeup. He always wished others would make advances to him. [Rorschach Research Exchange and Journal of Projective Techniques, 1947, p.240]
The association with (male) homosexuality likely got a boost from the term gay cat, used as far back as 1893 in American English for "young hobo," one who is new on the road, also one who sometimes does jobs.
"A Gay Cat," said he, "is a loafing laborer, who works maybe a week, gets his wages and vagabonds about hunting for another 'pick and shovel' job. Do you want to know where they got their monica (nickname) 'Gay Cat'? See, Kid, cats sneak about and scratch immediately after chumming with you and then get gay (fresh). That's why we call them 'Gay Cats'." [Leon Ray Livingston ("America's Most Celebrated Tramp"), "Life and Adventures of A-no. 1," 1910]
Quoting a tramp named Frenchy, who might not have known the origin. Gay cats were severely and cruelly abused by "real" tramps and bums, who considered them "an inferior order of beings who begs of and otherwise preys upon the bum -- as it were a jackal following up the king of beasts" [Prof. John J. McCook, "Tramps," in "The Public Treatment of Pauperism," 1893], but some accounts report certain older tramps would dominate a gay cat and employ him as a sort of slave. In "Sociology and Social Research" (1932-33) a paragraph on the "gay cat" phenomenon notes, "Homosexual practices are more common than rare in this group," and gey cat "homosexual boy" is attested in Noel Erskine's 1933 dictionary of "Underworld & Prison Slang" (gey is a Scottish variant of gay).
The "Dictionary of American Slang" reports that gay (adj.) was used by homosexuals, among themselves, in this sense at least since 1920. Rawson ["Wicked Words"] notes a male prostitute using gay in reference to male homosexuals (but also to female prostitutes) in London's notorious Cleveland Street Scandal of 1889. Ayto ["20th Century Words"] calls attention to the ambiguous use of the word in the 1868 song "The Gay Young Clerk in the Dry Goods Store," by U.S. female impersonator Will S. Hays, but the word evidently was not popularly felt in this sense by wider society until the 1950s at the earliest.
"Gay" (or "gai") is now widely used in French, Dutch, Danish, Japanese, Swedish, and Catalan with the same sense as the English. It is coming into use in Germany and among the English-speaking upper classes of many cosmopolitan areas in other countries. [John Boswell, "Christianity, Social Tolerance, and Homosexuality," 1980]
As a teen slang word meaning "bad, inferior, undesirable," without reference to sexuality, from 2000.