to a T

The origins of this phrase are uncertain, but it has been observed in print since at least 1693, and likely was around well before that. The possibly related phrase to a tittle is found in a 1607 play, The Woman Hater by Francis Beaumont and John Fletcher ("I'll quote him to a tittle"). The T in the phrase to a T is likely the first letter of a word, with tittle being the most likely source. * Other theories with little evidence point to golf tees, for their small size; this may have at least influenced the alternative form to a tee. Some speculate a relationship with T-square, a measuring device introduced around the turn of the century. Others claim the expression refers to the correct completion of the letter t by crossing it. *In print from "Two Years Before the Mast" published in 1840, and, even then, using quotes, refers to the practice of squaring up a yardarm with a mast on a sailing ship such that it made a perpendicular T.

来源：wiktionary