1705, "small (not capital) letter," from French minuscule (17c.), from Latin minuscula, in minuscula littera "slightly smaller letter," fem. of minusculus "rather less, rather small," diminutive of minus "less" (from PIE root *mei- (2) "small"). It refers to the kind of reduced alphabetical character which arose 7c. and was from about 9c. substituted in writing for the large uncial. From it the small or lower-case letters of the modern Latin alphabet were derived.
As an adjective, from 1727 in printing, "not capital, of reduced form, small" (of letters); the general sense of "extremely small" is attested by 1893. Related: Minuscular.