thymus n.

plant of the mint family, late 14c., from Old French thym, tym (13c.) and directly from Latin thymum, from Greek thymon, which had been derived from PIE root *dheu- (1), base of words meaning "smoke," for its scent or from being burned as a sacrifice, but Beekes finds this "doubtful" and suggests that "As a local plant name, the word is liable to be of Pre-Greek origin." Related: Thymic.

twentieth letter of the English alphabet; in the Phoenician alphabet the corresponding sign was the 22nd and last; everything after T in the modern alphabet represents European alterations or additions. The sound has been consistent throughout its history.

In Late Latin and Old French, -t- before -e- and -i- acquired the "s" value of -c- and words appeared in both spellings (nationem/nacionem) and often passed into Middle English with a -c- (nacioun). In most of these the spelling was restored to a -t- by or in the period of early Modern English, but sorting them out took time (Edmund Coote's "English Schoole-maister" (1596) noted malicious/malitious) and a few (space, place, coercion, suspicion) resisted the restoration. 

To cross one's t's(and dot one's i's) "to be exact" is attested from 1849. Phrase to a T "exactly, with utmost exactness" is recorded from 1690s, though the exact signification remains uncertain despite much speculation. The measuring tool called a T-square (sometimes suggested as the source of this) is recorded by that name only from 1785. The T-cell (1970) so called because they are derived from the thymus. As a medieval numeral, T represented 160. A T was formerly branded on the hand of a convicted thief.

nitrogenous base, 1894, from German (Kossel and Neumann, 1893), from thymic acid, from which it was isolated, the acid so called because obtained from the thymus gland. With chemical suffix -ine (2).