To a T (also, 'To a Tee') is a common English idiom meaning 'exactly' or 'perfectly'. Common examples of modern use include 'down to a T', 'suits to a T', and 'fits to a T'.
The phrase first appeared in writing in 1693, but it was almost certainly a shortening of an earlier phrase -- however, there is some debate as to what that phrase might have been.
The most popular theory is that is is a shortening of the phrase "to a tittle", first seen in writing in Francis Beaumont and John Fletcher's 1607 play, The Woman Hater. A 'tittle', in this case, is the technical term for the dot over the lowercase i or j. This origin is supported by the fact that 'jot and tittle' was a common phrase at the time, appearing in English translations of the Bible (specifically, Matthew 5:18) since 1526; in this case 'jot' is the name of the smallest letter of the Greek alphabet, iota, which has long been used to refer idiomatically to a small thing or…