aptitude n.

mid-14c., "inclined, disposed;" late 14c., "suited, fitted, adapted, possessing the necessary qualities for the purpose," from Old French ate "fitting, suitable, appropriate" (13c., Modern French apte), or directly from Latin aptus "fit, suited, proper, appropriate," adjectival use of past participle of *apere "to attach, join, tie to." This is reconstructed to be from PIE root *ap- (1) "to grasp, take, reach" (source also of Sanskrit apnoti "he reaches," Latin apisci "to reach after, attain," Hittite epmi "I seize"). The elliptical sense of "becoming, appropriate" is from 1560s.

1660s, "posture or position of a figure in a statue or painting," via French attitude (17c.), from Italian attitudine "disposition, posture," also "aptness, promptitude," from Late Latin aptitudinem (nominative aptitudo; see aptitude, which is its doublet).

Originally 17c. a technical term in art; later generalized to "a posture of the body supposed to imply some mental state" (1725). The sense of "a settled behavior reflecting feeling or opinion" is by 1837. The meaning "habitual mode of regarding" is short for attitude of mind (1757). Connotations of "antagonistic and uncooperative" developed by 1962 in slang.

Attitude is generally studied for the sake of looking graceful ; hence it is sometimes affected, the practice of it being then called attitudinizing. An attitude is often taken intentionally for the purpose of imitation or exemplification ; generally attitude is more artistic than posture. [Century Dictionary]

"unsuitableness, unfitness; unskilfulness, awkwardness," 1610s, from in- (1) "not, opposite of" + aptitude. The Frenchified version is ineptitude.