in music, stands for Tutti (all), meaning all the instruments or voices are to join. It is the opposite of S for Solo.
-t- inserted with a double hyphen between a verb ending with a vowel and the pronouns elle, il, or on, is called “t ephelcystic,” as, aime-t-il, dire-t-on. (See N, Marks In Grammar.)
Marked with a T. Criminals convicted of felony, and admitted to the benefit of clergy, were branded on the brawn of the thumb with the letter T (thief). The law was abolished by 7 and 8 George IV., c. 27.
It fits to a T. Exactly. The allusion is to work that mechanics square with a T-rule, especially useful in making right angles, and in obtaining perpendiculars on paper or wood.
The saintly T's. Sin Tander, Sin Tantony, Sin Tawdry, Sin Tausin, Sin Tedmund, and Sin Telders; otherwise St. Andrew, St. Anthony, St. Audry, St. Austin [Augustine], St. Edmund, and St. Ethelred. Tooley is St. Olaf.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894