The English adjective imbecile is, via French, from the Latin imbecillus, or imbecillis, meaning weak, feeble, in body or mind.
In his etymological encyclopaedia Originum sive Etymologiarum (The Origins or Etymologies), the Spanish archbishop and Doctor of the Church St Isidore of Seville (circa 560-636) wrote that the literal meaning of the Latin adjective is quasi sine baculo, as though (walking) without a supporting staff.
The original form of the adjective would therefore have been an assumed imbacillus, composed of the prefix in-, expressing negation, and bacillum, or bacillus, a diminutive of baculum, or baculus, a supporting staff, and its original sense tottering for lack of support. This etymology, long regarded as an early myth, is now accepted by competent authorities.
(Similarly, the adjective imbellis, based on bellum, war, meant unwarlike, unfit for war, peaceful.)
The English adjective appeared in the sense physically weak or impotent in The Complaynt of Scotland, written in 1548:
The solist ande attentiue laubirs that i tuke to vrit thir passagis befor rehersit, gart al my body be cum imbecille ande verye.
(The anxious and attentive exertions that it took me to write these passages before they were uttered have caused all my body to become imbecile and exhausted.)
The English lexicographer Samuel Johnson (1709-84) defined imbecile as meaning “weak; feeble; wanting strength of either mind or body” in A Dictionary of the English Language (1755).
This additional sense mentally weak, of weak character or will through want of mental power, gave rise to the current sense stupid, idiotic. The following by the English essayist Thomas De Quincey (1785-1859) illustrates this sense development. In Glance at the works of Mackintosh, published in Tait’s Edinburgh Magazine of July 1846, he wrote, about King Christian 7th of Denmark:
He had the misfortune to be “imbecile,” which is a word of vague meaning; in fact, he was partially an idiot, and, at times, a refractory madman.
The first known user of imbecile as a noun meaning a stupid person is General Nugent in a letter dated 19th November 1802, written in Jamaica to the marquis of Buckingham:
Le Clerc was an imbecile; but he is no more. His corpse, with Madame Le Clerc, are gone to France in the ‘Swiftsure.’
The same sense development took place in French. For example, in Antigone, ou La piété (1580), the French tragic dramatist Robert Garnier (1544-90) used the expression le sexe imbécile to mean women, imbécile still having its original sense physically weak.