masturbation (n.)

"of or pertaining to the hand; done, made, or used by hand;" c. 1400, from Latin manualis "of or belonging to the hand; that can be thrown by hand," from manus "hand, strength, power over; armed force; handwriting," from PIE root *man- (2) "hand." The military manual exercise (1760) is "the art of handling the rifle and other arms with precision and according to the prescribed method."

1540s, "mentally slow, lacking ordinary activity of mind, dull, inane," from French stupide (16c.) and directly from Latin stupidus "amazed, confounded; dull, foolish," literally "struck senseless," from stupere "be stunned, amazed, confounded," from PIE *stupe- "hit," from root *(s)teu- (1) "to push, stick, knock, beat" (see steep (adj.)). Related: Stupidly; stupidness.

Native words for this idea include negative compounds with words for "wise" (Old English unwis, unsnotor, ungleaw), also dol (see dull (adj.)), and dysig (see dizzy (adj.)). Stupid retained its association with stupor and its overtones of "stunned by surprise, grief, etc." into mid-18c. The difference between stupid and the less opprobrious foolish roughly parallels that of German töricht vs. dumm but does not exist in most European languages.

Men are born ignorant, not stupid; they are made stupid by education. [Bertrand Russell, paraphrasing Helvétius]