immature adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

c. 1400, maturen, "encourage suppuration;" mid-15c., of plants, "cause to ripen, bring to maturity," from Latin mātūrare "to ripen, bring to maturity," from mātūrus "ripe, timely, early," related to māne "early, of the morning," from PIE *meh-tu- "ripeness." De Vaan writes that "The root is probably the same as in mānus 'good'." Intransitive sense of "come to a state of ripeness, become ripe or perfect" is from 1650s. The financial sense of "reach the time for payment" is by 1861. Related: Matured; maturing.

1530s, "untimeliness," from Latin immaturitatem (nominative immaturitas) "unripeness," from immaturus "unripe, untimely" (see immature). Meaning "lack of maturity" attested from c. 1600.