Online Etymology Dictionary

klutzy (adj.): 1965, from klutz + -y (2). Related: Klutziness.
kn-: Germanic consonant cluster; the sound is still evident in most sister languages but in English it has been reduced to "n-" in standard pronunciation since before 1750, and for about a century before that it had been pronounced hn-, dn-, tn-. It was fully voiced in Old and Middle English.
knack (n.): mid-14c., "deception, trick, device," of uncertain origin, probably from a Low German word meaning "a sharp sounding blow" (cf. Middle English knak, late 14c.; German knacken "to crack"), of imitative origin. Sense of "special skill" is first recorded 1580s, if this is in fact the same word.
knacker (v.): usually in past tense, knackered, "to kill, castrate" (1855), but most often used in weakened sense of "to tire out" (1883); apparently from knacker (n.) "worn-out or useless horse," 1812, of unknown origin; possibly from a dialectal survival of a Scandinavian word represented by Old Norse hnakkur "saddle," hnakki "back of the neck," and thus possibly related to neck (n.).
knackered (adj.): "worn out, tired," past participle adjective from knacker (v.).
knap (v.): "to strike with a sharp sound," late 15c., echoic. Earlier (c.1400) as a noun meaning "abrupt stroke." Related: Knapped; knapping.
knapsack (n.): c.1600, from Low German Knapsack (Dutch knapzak), probably from knappen "to eat" literally "to crack, snap" + Sack "bag" (see sack (n.1)).
knave (n.): Old English cnafa "boy, male servant," common Germanic (cf. Old High German knabo "boy, youth, servant," German knabe "boy, lad," also probably related to Old English cnapa "boy, youth, servant," Old Norse knapi "servant boy," Dutch knaap "a youth, servant," Middle High German knappe "a young squire," German Knappe "squire, shield-bearer"). The original meaning might have been "stick, piece of wood" [Klein]. Sense of "rogue, rascal" first recorded c.1200. In playing cards, "the jack," 1560s.
knavery (n.): 1520s, from knave + -ery.
knavish (adj.): late 14c., from knave + -ish. Related: Knavishly; knavishness.
knead (v.): Old English cnedan "to knead," from Proto-Germanic *knedanan (cf. Old Saxon knedan, Middle Dutch cneden, Dutch kneden, Old High German knetan, German kneten, Old Norse knoða "to knead"). Originally a strong verb (past tense cnæd, past participle cneden).
knee (n.): Old English cneo, cneow "knee," from Proto-Germanic *knewam (cf. Old Norse kne, Old Saxon kneo, Old Frisian kni, Middle Dutch cnie, Dutch knie, Old High German kniu, German Knie, Gothic kniu), from PIE root *g(e)neu- (cf. Sanskrit janu, Avestan znum, Hittite genu "knee;" Greek gony "knee," gonia "corner, angle;" Latin genu "knee"). Knee-slapper "funny joke" is from 1955.
knee (v.): early 13c., "to bend the knee, kneel," from Old English cneowian, from cneow (see knee (n.)). The meaning "to strike with the knee" is first recorded 1892. Related: Kneed; kneeing.
knee-deep (adj.): 1530s, from knee (n.) + deep (adj.).
knee-high (adj.): 1743, from knee (n.) + high (adj.). Phrase knee-high to a grasshopper first recorded 1851 (earliest form was knee-high to a toad, 1814).
knee-jerk (n.): patellar reflex, neurological phenomenon discovered and named 1876; the figurative use appeared soon after the phrase was coined.
kneecap (n.): 1650s, "a covering or protection for the knee," from knee (n.) + cap (n.). Meaning "bone in front of the knee joint" is from 1869; the verb in the underworld sense of "to shoot (someone) in the knee" as punishment is attested by 1975. Related: Kneecapped.
kneel (v.): Old English cneowlian, from cneow (see knee); cf. Middle Low German knelen, Middle Dutch cnielen, Dutch knielen Gothic knussjan. Past tense knelt is a modern formation (19c.) on analogy of feel/felt, etc. Related: Kneeling.
knell (n.): Old English cnyll "sound made by a bell when struck or rung slowly," perhaps of imitative origin. The Welsh cnull "death-bell" appears to be a borrowing from English. For vowel evolution, see bury.
knell (v.): Old English cnyllan "to toll a bell; strike, knock," cognate with Middle High German erknellen "to resound," Old Norse knylla "to beat, thrash;" probably imitative. Related: Knelled; knelling.
Knesset: Israeli parliament, 1949, from Mishnaic Hebrew keneseth "gathering, assembly," from stem of Hebrew kanas "he gathered, assembled, collected."
knew: Old English cneow, past tense of know (v.).
knick-knack (n.): also knickknack, 1570s, reduplication of knack "stratagem, trick."
Knickerbocker: "descendant of Dutch settlers of New York," 1831, from Diedrich Knickerbocker, the name under which Washington Irving published his popular "History of New York" (1809). The pen-name was borrowed from Irving's friend Herman Knickerbocker, and literally means "toy marble-baker."
knickers (n.): "short, loose-fitting undergarment," now usually for women but not originally so, 1866, shortening of knickerbockers (1859), said to be so called for their resemblance to the trousers of old-time Dutchmen in Cruikshank's illustrations for Washington Irving's "History of New York" (see knickerbocker).
knife (n.): late Old English cnif, probably from Old Norse knifr, from Proto-Germanic *knibaz (cf. Middle Low German knif, Middle Dutch cnijf, German kneif), of uncertain origin. To further confuse the etymology, there also are forms in -p-, e.g. Dutch knijp, German kneip. French canif "penknife" (mid-15c.) is borrowed from Middle English or Norse.
knife (v.): 1865, from knife (n.). Related: Knifed; knifing.
knight (n.): Old English cniht "boy, youth; servant, attendant," common West Germanic (cf. Old Frisian kniucht, Dutch knecht, Middle High German kneht "boy, youth, lad," German Knecht "servant, bondman, vassal"), of unknown origin. The plural in Middle English sometimes was knighten. Meaning "military follower of a king or other superior" is from c.1100. Began to be used in a specific military sense in Hundred Years War, and gradually rose in importance until it became a rank in the nobility 16c. The chess piece so called from mid-15c. Knight in shining armor in figurative sense is from 1917, from the man who rescues the damsel in distress in romantic dramas (perhaps especially "Lohengrin"). Knights of Columbus, society of Catholic men, founded 1882 in New Haven, Connecticut, U.S.; Knights of Labor, trade union association, founded in Philadelphia, 1869; Knights of Pythias, secret order, founded in Washington, 1864.
knight (v.): "to make a knight of (someone)," early 13c., from knight (n.). Related: Knighted; knighting.
knighthood (n.): Old English cnihthad "the period between childhood and manhood;" see knight (n.) + -hood. Sense of "rank or dignity of a knight" is from c.1300, and probably is an independent formation.
knightly (adj.): Old English cnihtlic; see knight (n.) + -ly (1).
knish (n.): 1930, from Yiddish, from Russian knysh, a kind of cake.
knit (v.): Old English cnyttan "to tie with a knot, bind, fasten," related to Old Norse knytja "bind together," Middle Low German knütten "to tie, knot," Old English cnotta "a knot," from Proto-Germanic *knuttjan, from stem *knutt-. Of brows, late 14c. Meaning "to do knitting" (especially plain stitch) is from 1520s. Related: Knitted; knitting.
knitter (n.): mid-15c., agent noun from knit (v.).
knitting (n.): "knitted work," 1848, from present participle of knit (v.). Knitting-needle is from 1590s.
knob (n.): late 14c., knobe, probably from a Scandinavian or German source (cf. Middle Low German knobbe "knob," Middle Dutch cnoppe, Dutch knop, Old Frisian knopp, knapp, Old High German knopf, German Knopf "button," Old Norse knyfill "short horn"). Meaning "knoll, isolated round hill" is first recorded 1640s, especially in U.S.
knobby (adj.): 1540s, from knob + -y (2). Alternative form knobbly attested from 1859. Related: Knobbiness.
knock (v.): Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Meaning "deprecate, put down" is from 1892. Related: Knocked; knocking. Knock-kneed first attested 1774. Knock-down, drag-out is from 1827. Command knock it off "stop it" is first recorded 1880, perhaps from auctioneer's term for "dispose of quickly:"
At the commencement of the sales, he gave every one that wanted to purchase a paper containing a description of the lands that were to be sold; and, as the sales were cried, he called over the numbers and described the land; and when it got up to one dollar and a quarter an acre, if no body bid, after it was cried two or three times, he would say, knock it off, knock it off. [U.S. Senate record, 1834]
knock (n.): mid-14c., from knock (v.). As an engine noise, from 1899.
knock off (v.): "desist, stop," 1640s; "do hastily," 1817; "kill, murder," 1919; from knock (v.) + off.
knock up (v.): 1660s in sense of "arouse by knocking at the door," from knock (v.). However it is little used in this sense in American English, where the phrase means "get a woman pregnant" (1813), possibly ultimately from knock "to copulate with" (1590s; cf. slang knocking-shop "brothel," 1860).
Knocked up in the United States, amongst females, the phrase is equivalent to being enciente, so that Englishmen often unconsciously commit themselves when amongst our Yankee cousins. [John Camden Hotten, "The Slang Dictionary," London, 1860]
knocker (n.): late 14c., agent noun from knock. Sense of "door banger" is by 1590s. Knockers "a woman's breasts" is slang attested from 1941.
knockoff (n.): "cheap imitation," 1966, from the verbal phrase knock off "do hastily;" in reference to the casual way the things are made.
knockout (n.): also knock-out, in fighting, 1887, from verbal phrase knock out "to stun by a blow for a 10-count" in boxing, short for to knock out of time; see knock (v.) + out. Slang meaning "attractive person" is from 1892. To knock oneself out "make a great effort" is from 1936.
knoll (n.): Old English cnoll "hilltop, small hill, clod, ball," related to Old Norse knollr "hilltop;" German knolle "clod, lump;" Dutch knol "turnip," nol "a hill."
knot (n.): Old English cnotta "intertwining of ropes, cords, etc.," from Proto-Germanic *knuttan- (cf. Low German knütte, Old Frisian knotta "knot," Dutch knot, Old High German knoto, German Knoten, perhaps also Old Norse knutr "knot, knob"). Figurative sense of "difficult problem" was in Old English (cf. Gordian knot). Symbolic of the bond of wedlock, early 13c. As an ornament of dress, first attested c.1400. Meaning "thickened part or protuberance on tissue of a plant" is from late 14c. The nautical unit of measure (1630s) is from the practice of attaching knotted string to the log line. The ship's speed can be measured by the number of knots that play out while the sand glass is running.
The distance between the knots on the log-line should contain 1/120 of a mile, supposing the glass to run exactly half a minute. [Jorge Juan and Antonio de Ulloa, "A Voyage to South America" 1760]
knot (v.): "to tie in a knot," mid-15c., from knot (n.). Related: Knotted (late 12c.), knotting.
knothead (n.): "stupid person," by 1899, American English, from knot (n.) + head (n.). Joe Knothead is the name of a character in an 1857 blackface satire publication. And a local history from Massachusetts published in 1879 describes an old-time character known as knot-head because "[d]uring the hottest days of summer ... he worked bare-headed in the sun ...."

Knothead also was used as a term in cattle and sheep raising, defined in 1922 as "a type of poorly bred, stunted northern cattle, about the size of yearlings, but with heavy horns indicating that they are older." It turns up, however, in an 1849 petition to the Ohio Legislature, recommending a certain person for a court position, in part because he is a knot-head, which the report of the petition notes is a term of praise for a judge because they are asked to untangle knotty legal questions, but which phrase, it adds, "is believed not to be in use among gentlemen in the north part of the State." [Appendix to the Journal of the Ohio House of Representatives, Session of 1848-9]
knothole (n.): 1726, see knot (n.) + hole (n.).
knotty (adj.): mid-13c. (figurative use early 13c.), from knot (n.) + -y (2). Related: Knottiness.