Английский Этимологический словарь - tit (2)

- 1548, originally "any small animal or object" (as in compound forms such as titmouse, tomtit, etc.), also originally used of small horses. Used figuratively of persons after 1734, but earlier for "a girl or young woman," usually in deprecatory sense of "a hussy, minx" (1599). Tit for tat (1556) is possibly an alteration of tip for tap "blow for blow," from tip (4) "tap" + tap "touch lightly."