For two centuries, an imbecile in English was simply someone weak in body. The insult came much later, and its route ran through medicine.
English borrowed the adjective in the 1540s from French imbecile, "weak, feeble." The French came from Latin imbecillus, with the same meaning. Word Histories finds the first English use in The Complaynt of Scotland, in 1548, for a writer whose body had become "imbecille" from labour.
Where the Latin word came from is the disputed part. The traditional answer splits it into in-, "not," and baculum, "a staff." An imbecile would then be someone tottering without a stick to lean on. An old Latin gloss says exactly that: quasi sine baculo, "as though without a staff." Etymonline credits the gloss to Juvenal, while Word Histories attributes it to Isidore of Seville.
Scholars disagree sharply about whether to believe it. Etymonline calls the origin uncertain. It reports that the Century Dictionary found the staff theory improbable. The Latin scholar de Vaan called it "very far-fetched," since people who walk unsupported are the stronger ones. Word Histories takes the opposite view, saying the etymology, once dismissed as a myth, is "now accepted by competent authorities." Neither side has settled it.
The meaning shifted from the body to the mind in the mid-1700s. Samuel Johnson's 1755 Dictionary defined it as weak in mind or body. The noun, for a feeble-minded person, appears in 1802, in a letter from Jamaica by General Nugent describing Le Clerc as "an imbecile."
In the early 1900s the word became a clinical grade. Henry H. Goddard proposed a scale based on mental age, with "imbecile" between "idiot" below and "moron" above. Sources differ on the range: Wikipedia gives a mental age of three to seven, Etymonline roughly six to nine. All three labels are now considered archaic and offensive when applied to people with disabilities.
That medical past is why the word stings. It began as a description of a frail body and became a diagnosis before it became an insult.