Online Etymology Dictionary

titter (v.): 1610s, "giggle in a suppressed or covert way," probably of imitative origin. Related: Tittered; tittering. The noun is first recorded 1728.
titties (n.): 1746 (plural); see tit (n.1) + -ie.
tittle (n.): late 14c., "small stroke or point in writing," representing Latin apex in Late Latin sense of "accent mark over a vowel," borrowed (perhaps by influence of Provençal titule "the dot over -i-") from Latin titulus "inscription, heading" (see title (n.)).
titular (adj.): 1590s, perhaps by influence of Middle French titulaire, from Latin titulus (see title).
tix (n.): short for tickets, by 1947, originally headlinese.
tizzy (n.): 1935, American English colloquial, of uncertain origin, perhaps related to slang tizzy "sixpence piece" (1804), a corruption of tester, a name for the coin (see tester (n.2)).
tlc: by 1953 as an abbreviation of tender loving care.
Tlingit: Indian group in Alaska and Canada, the people's word for themselves, literally "human beings."
tmesis: 1570s, from Greek tmesis "a cutting," related to temnein "to cut," tome "a cutting" (see tome). The separation of the elements of a compound word by the interposition of another word or words (e.g. a whole nother).
TNT: 1915, abbreviation of trinitrotoluene (1908).
to (prep.): Old English to "in the direction of, for the purpose of, furthermore," from West Germanic *to (cf. Old Saxon and Old Frisian to, Dutch too, Old High German zuo, German zu "to"), from PIE pronomial base *do- "to, toward, upward" (cf. Latin donec "as long as," Old Church Slavonic do "as far as, to," Greek suffix -de "to, toward," Old Irish do, Lithuanian da-).

In Old English, the preposition (go to town) leveled with the adverb (the door slammed to) except where the adverb retained its stress (tired and hungry too); there it came to be written with -oo (see too).

The nearly universal use of to with infinitives (to sleep, to dream, etc.) arose in Middle English out of the Old English dative use of to, and it helped drive out the Old English inflectional endings (though in this use to itself is a mere sign, without meaning).

Commonly used as a prefix in Middle English (to-hear "listen to," etc.), but few of these survive (to-do, together, and time references such as today, tonight, tomorrow -- Chaucer also has to-yeere). To and fro "side to side" is attested from mid-14c. Phrase what's it to you "how does that concern you?" goes back a long way:
Huæd is ðec ðæs?
[John xxi:22, in Lindisfarne Gospel, c.950]
to-: particle expressing separation, from West Germanic *ti- (cf. Old Frisian ti-, Old High German zi-, German zer-), from Proto-Germanic *tiz-, cognate with Latin-derived dis-. Some 125 compound verbs with this element are recorded in Old English; their number declined rapidly in Middle English and disappeared by c.1500 except as conscious archaisms.
to-do (n.): 1570s, from the verb phrase to do, from Old English to don "proper or necessary to be done" (see to). Meaning "disturbance, fuss" is first recorded 1827.
toad (n.): Old English tadige, tadie, of unknown origin and with no known cognates outside English.
toadstone (n.): "stone or stone-like object, supposedly magical (with healing or protective power) and found in the heads of certain toads," is attested from 1550s, from toad + stone (n.). Translating Greek batrakhites, Medieval Latin bufonites; cf. also French crapaudine (13c.), German krötenstein.
toadstool (n.): late 14c., apparently just what it looks like: a fanciful name from Middle English tadde "toad" (see toad) + stole "stool" (see stool). Toads themselves were regarded as highly poisonous, and this word is popularly restricted to inedible or poisonous fungi, as opposed to mushrooms (e.g. toad-cheese, a poisonous fungi).
toady (n.): "servile parasite," 1826, apparently shortened from toad-eater "fawning flatterer" (1742), originally referring to the assistant of a charlatan, who ate a toad (believed to be poisonous) to enable his master to display his skill in expelling the poison (1620s). The verb is recorded from 1827. Related: Toadied; toadying.
toast (v.1): "to brown with heat," late 14c., from Old French toster "to toast or grill" (12c.), from Vulgar Latin *tostare (source of Italian tostare, Spanish tostar), frequentative of Latin torrere (past participle tostus) "to parch" (see terrain). Related: Toasted; toasting.
toast (v.2): "to propose or drink a toast," 1700, from toast (n.1). This probably is the source of the Jamaican and U.S. black word meaning "extemporaneous narrative poem or rap" (1962). Related: Toasted; toasting.
toast (n.1): "a call to drink to someone's health," 1700 (but said by Steele, 1709, to date to the reign of Charles II), originally referring to the beautiful or popular woman whose health is proposed and drunk, from the use of spiced toast (n.2) to flavor drink, the lady regarded as figuratively adding piquancy to the wine in which her health was drunk.
toast (n.2): "a toasted piece of bread," early 15c., from toast (v.1); slang meaning "a goner, person or thing already doomed or destroyed" is recorded by 1987, perhaps from notion of computer circuits being "fried," and with unconscious echoes of earlier figurative phrase to be had on toast (1886) "to be served up for eating."
toaster (n.): 1580s, agent noun from toast (v.1). Electrical type is from 1913. In reference to a person who proposes or pledges a drinking toast, from 1704 (from toast (v.2)).
toasty (adj.): "warm and comfortable," 1890, from toast (v.1) + -y (2). Related: Toastiness.
tobacco (n.): 1580s, from Spanish tabaco, in part from an Arawakan (probably Taino) language of the Caribbean, said to mean "a roll of tobacco leaves" (according to Las Casas, 1552) or "a kind of pipe for smoking tobacco" (according to Oviedo, 1535). Scholars of Caribbean languages lean toward Las Casas' explanation. But Spanish tabaco (also Italian tabacco) was a name of medicinal herbs from early 15c., from Arabic tabbaq, attested since 9c. as the name of various herbs. So the word may be a European one transferred to an American plant.

Cultivation in France began 1556 with an importation of seed by Andre Thevet; introduced in Spain 1558 by Francisco Fernandes. Tobacco Road as a mythical place representative of rural Southern U.S. poverty is from the title of Erskine Caldwell's 1932 novel.
tobacconist (n.): "dealer in tobacco," 1650s, from tobacco + -ist + abnormal inserted consonant; earlier meaning was "person addicted to tobacco" (1590s).
Tobias: masc. proper name, from Late Latin Tobias, from Greek Tobias, from Hebrew Tobhiyyah, literally "the Lord is my Good," from Hebrew tobh "good." Toby is a short form.
toboggan (n.): "long, flat-bottomed sled," 1829, from Canadian French tabagane, from Algonquian (probably Micmac) tobakun "a sled." The verb is recorded from 1846. As American English colloquial for a type of long woolen cap, it is recorded from 1929 (earlier toboggan cap, 1928), presumably because one wore such a cap while tobogganing.
Toby: familiar form of masc. proper name Tobias, in various colloquial usages, e.g. "jug" (1840), "drinking mug in the form of a stout old man;" as a type of collar (1882) it refers to that worn by the dog Toby in the 19c. Punch and Judy shows. Also in Toby show (by 1942, American English) "comedy act based on the stock character of a boisterous, blundering yokel."
toc: word used for the letter T in radio communication, 1898.
toccata (n.): 1724, from Italian toccata, from toccare "to touch." A composition for keyboard instrument, intended to exhibit the touch and technique of the performer, and having the air of an improvisation.
Tocharian: in reference to an extinct people and Indo-European language of Chinese Turkestan, 1927, from French tocharien, from Greek Tokharoi (Strabo), name of an Asiatic people who lived in the Oxus valley in ancient times. Earlier Tocharish (1910), from German tocharisch. The identification of this culture with the people named by Strabo was suggested in 1907 by F.W.K. Müller and "is obviously erroneous" (Klein).
tocsin (n.): "alarm bell," 1580s, from Middle French toquassen "an alarm bell, the ringing of an alarm bell" (late 14c.), from Old Provençal tocasenh, from tocar "to strike" (from Vulgar Latin *toccare "strike a bell;" see touch) + senh "bell, bell note," from Late Latin signum "bell, ringing of a bell," in Latin "mark, signal." The current English spelling is from 1794, adopted from modern French.
today (n.): Old English todæge, to dæge "on (the) day," from to "at, on" (see to) + dæge, dative of dæg "day" (see day). Generally written as two words until 16c., after which it usually was written to-day until early 20c.

Similar constructions exist in other Germanic languages (cf. Dutch van daag "from-day," Danish and Swedish i dag "in day"). German heute is from Old High German hiutu, from Proto-Germanic *hiu tagu "on (this) day," with first element from PIE pronomial stem *ki-, represented by Latin cis "on this side."
Todd: masc. proper name, also a surname (late 12c.), from Middle English todde "fox," a Northern English word of unknown origin.
toddle (v.): "to run or walk with short, unsteady steps," c.1600, Scottish and northern British, of uncertain origin, possibly related to totter (1530s); an earlier sense of "to toy, play" is found c.1500. Related: Toddled; toddling.
toddler (n.): 1793, agent noun from toddle.
toddy (n.): 1610s, alteration of taddy (1610s), tarrie (c.1600) "beverage made from fermented palm sap," from Hindi tari "palm sap" (in which the -r- sounds close to an English -d-), from tar "palm tree," from Sanskrit tala-s, probably from a Dravidian language (cf. Kannada tar, Telugu tadu). Meaning "beverage made of alcoholic liquor with hot water, sugar, and spices" first recorded 1786.
toe (n.): Old English ta (plural tan), contraction of *tahe (Mercian tahæ), from Proto-Germanic *taikhwo (cf. Old Norse ta, Old Frisian tane, Middle Dutch te, Dutch teen, Old High German zecha, German Zehe "toe"), probably originally meaning "fingers" as well (many PIE languages still use one word to mean both fingers and toes). The Old English plural tan survived in southwestern England to 14c. To be on (one's) toes "alert, eager" is recorded from 1921.
toe (v.): "touch or reach with the toes," 1813, from toe (n.). First recorded in expression toe the mark, which seems to be nautical in origin.
The chief mate ... marked a line on the deck, brought the two boys up to it, making them 'toe the mark.' [R.H. Dana, "Two Years Before the Mast," 1840]
Related: Toed; toeing.
toenail (n.): also toe-nail, 1735, from toe (n.) + nail (n.).
toff (n.): lower-class British slang for "stylish dresser, member of the smart set," 1851, said to be probably an alteration of tuft, formerly an Oxford University term for a nobleman or gentleman-commoner (1755), in reference to the gold ornamental tassel worn on the caps of undergraduates at Oxford and Cambridge whose fathers were peers with votes in the House of Lords.
toffee (n.): 1825, tuffy, toughy, southern British dialectal variant of taffy. Modern spelling first recorded 1862.
tofu (n.): soy bean curd, 1880, from Japanese tofu, from Chinese doufu, from dou "beans" + fu "rotten."
tog (n.): 1708, "any outer garment," shortened from togman "cloak, loose coat" (1560s), thieves' cant word, formed from French togue "cloak," from Latin toga (see toga). Middle English toge "toga" (14c.) was also a cant word for "coat."
toga (n.): c.1600, from Latin toga "cloak or mantle," related to tegere "to cover" (see stegosaurus).

The outer garment of a Roman citizen in time of peace; toga prætexta had a broad purple border and was worn by children, magistrates, persons engaged in sacred rites, and later also emperors; toga virilis, the "toga of manhood," was assumed by boys at puberty.

Breeches, like the word for them (Latin bracae) were alien to the Romans, the dress of Persians, Germans and Gauls, so that bracatus "wearing breeches" was a term in Roman geography meaning "north of the Alps." College fraternity toga party popularized by movie "Animal House" (1978), but this is set in 1962.
together: Old English togædere, from to (see to) + gædere "together" (adv.), apparently a variant of the adverb geador "together," related to gadrian (see gather).

German cognate zusammen substitutes second element with Old High German verbal cognate of English same (Old English also had tosamne "together"). Adjective meaning "self-assured, free of emotional difficulties" is first recorded 1966.
togetherness (n.): 1650s, "state of being together," from together + -ness. Sense of "fellowship, fellow-feeling," is from 1930.
toggery (n.): "clothes collectively," 1812, from tog + -ery.
toggle (n.): 1769, "short pin passed through the eye of a rope," a nautical word of uncertain origin, perhaps a frequentative form of tog "tug." Meaning "a kind of wall fastener" is recorded from 1934. Toggle bolt is from 1794; toggle switch first attested 1938.
toggle (v.): 1836, from toggle (n.). Related: Toggled; toggling.