imbecility (n.) Look up imbecility at
early 15c., "physical weakness, feebleness (of a body part), impotence," from Middle French imbécillité and directly from Latin imbecillitatem (nominative imbecillitas) "weakness, feebleness, helplessness," from imbecillus "weak, feeble," of uncertain origin. "Weakness in mind" (as opposed to body) was a secondary sense in Latin but was not attested in English until 1620s.

The Latin word traditionally is said to mean "unsupported" or "without a walking stick" (Isidore: quasi sine baculo), from assimilated form of in- "not, opposite of" (see in- (1)) + baculum "a stick" (see bacillus), but Century Dictionary finds that "improbable" and de Vaan adds "it seems to me that exactly the persons who can walk without a support are the stronger ones."