early 15c., "tendency, likelihood," from Late Latin aptitudo (genitive aptitudinis) "fitness," noun of quality from Latin aptus "joined, fitted" (see apt). Meaning "natural capacity to learn" is 1540s; that of "state or quality of being fit (for a purpose or position)" is from 1640s. Related: Aptitudinal. A doublet of attitude.
updated on January 31, 2017