From The Jargon File (version 4.4.7, 29 Dec 2003):
T
/T/
1. [from LISP terminology for ‘true’] Yes. Used in reply to a question
(particularly one asked using The -P convention). In LISP, the constant T
means ‘true’, among other things. Some Lisp hackers use ‘T’ and ‘NIL’
instead of ‘Yes’ and ‘No’ almost reflexively. This sometimes causes
misunderstandings. When a waiter or flight attendant asks whether a hacker
wants coffee, he may absently respond ‘T’, meaning that he wants coffee;
but of course he will be brought a cup of tea instead. Fortunately, most
hackers (particularly those who frequent Chinese restaurants) like tea at
least as well as coffee — so it is not that big a problem.
2. See time T (also since time T equals minus infinity).
3. [techspeak] In transaction-processing circles, an abbreviation for the
noun ‘transaction’.
4. [Purdue] Alternate spelling of tee.
5. A dialect of LISP developed at Yale. (There is an intended allusion to
NIL, “New Implementation of Lisp”, another dialect of Lisp developed for
the VAX)