What's the difference between i.e. and e.g.?
early 15c., "tendency, likelihood," from Middle French aptitude (14c.) or directly 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 "quality of being fit (for a purpose or position)" is from 1640s.