small, active bird, early 14c., titmose, from tit (n.2), expressing something small, + Old English mase "titmouse," from Proto-Germanic *maison (source also of Dutch mees, German meise), from adj. *maisa- "little, tiny." Spelling influenced 16c. by unrelated mouse, "when mose had long been obsolete as an independent word" [OED]. The proper plural is titmouses.
popular name of the American black-capped titmouse, 1834, American English, more or less echoic of its cry. The end of the call does sound like dee-dee-dee, but the opening is more of a chirp or twitter or tweet, so there may be some folk etymology in the word.
familiar shortening of masc. proper name Thomas, used by late 14c. as a type of a nickname for a common man (as in Tom, Dick, and Harry, 1734). Applied 17c. as a nickname for several exceptionally large bells. Short for Uncle Tom in the sense of "black man regarded as too servile to whites" is recorded from 1959.
Tom Walker, U.S. Southern colloquial for "the devil" is recorded from 1833. Tom and Jerry is first attested 1828 and later used in many extended senses, originally were the names of the two chief characters (Corinthian Tom and Jerry Hawthorn) in Pierce Egan's "Life in London" (1821); the U.S. cat and mouse cartoon characters debuted 1940 in "Puss Gets the Boot." Tom Thumb (1570s) was a miniature man in popular tradition before P.T. Barnum took the name for a dwarf he exhibited. Tom-tit "titmouse" is from 1709. Compare tomcat.