Imbecile comes from a Latin word bacillum, prefaced by the negation in: without a rod, or without a staff. Specifically: "Old French imbecile, from Latin imbecillus, 'without support,' feeble: in- not + bacillum, diminutive of baculum, staff, rod."  [American Heritage Dictionary)

The OED includes these notations:

1. In general sense, weak, feeble; esp. feeble of body, physically weak or impotent.

2. Mentally weak; of weak character or will through want of mental power; hence Fatuous, stupid, idiotic. (The chief current use)

3.  One who is an imbecile; a person of weak intellect.

See also Alice's comment on Joyce's use of this word.