tityoudaoicibaDictYouDict[tit 词源字典]
tit: English has three separate words tit. The oldest, ‘breast’ [OE], belongs to a West Germanic family of terms for ‘breast’ or ‘nipple’ that also includes German zitze and Dutch tit: it presumably originated in imitation of a baby’s sucking sounds. From Germanic it was borrowed into the Romance languages, giving Italian tetta, Spanish teta, Romanian tata, and French tette.

The Old French ancestor of this, tete, gave English teat [13], which gradually replaced tit as the ‘polite’ term. (Titillate [17] may be ultimately related). Tit the bird [18] is short for titmouse [14]. This in turn was formed from an earlier and now defunct tit, used in compounds denoting ‘small things’ and probably borrowed from a Scandinavian language, and Middle English mose ‘titmouse’, which came from a prehistoric Germanic *maisōn (source also of German meise and Dutch mees ‘tit’).

And the tit [16] of tit for tat (which produced British rhyming slang titfer ‘hat’ [20]) originally denoted a ‘light blow, tap’, and was presumably of onomatopoeic origin. (The tit- of titbit [17], incidentally, is probably a different word. It was originally tid- – as it still is in American English – and it may go back ultimately to Old English tiddre ‘frail’.)

=> teat, titillate; titmouse[tit etymology, tit origin, 英语词源]
tit (n.1)youdaoicibaDictYouDict
"breast," Old English titt "teat, nipple, breast" (a variant of teat). But the modern slang tits (plural), attested from 1928, seems to be a recent reinvention, used without awareness of the original form, from teat or from dialectal and nursery diminutive variant titties (pl.).
tit (n.2)youdaoicibaDictYouDict
1540s, a word used for any small animal or object (as in compound forms such as titmouse, tomtit, etc.); also used of small horses. Similar words in related senses are found in Scandinavian (Icelandic tittr, Norwegian tita "a little bird"), but the connection and origin are obscure; perhaps, as OED suggests, the word is merely suggestive of something small. Used figuratively of persons after 1734, but earlier for "a girl or young woman" (1590s), often in deprecatory sense of "a hussy, minx."