The Collaborative International Dictionary of English v.0.48:

Corporal \Cor"po*ral\ (k[^o]r"p[-o]*ral), n. [Corrupted fr. F.
   caporal, It. caporale, fr. capo head, chief, L. caput. See
   Chief, and cf. Caporal.] (Mil.)
   A noncommissioned officer, next below a sergeant. In the
   United States army he is the lowest noncommissioned officer
   in a company of infantry. He places and relieves sentinels.
   [1913 Webster]

   Corporal's guard, a detachment such as would be in charge
      of a corporal for guard duty, etc.; hence, derisively, a
      very small number of persons.

   Lance corporal, an assistant corporal on private's pay.
      --Farrow.

   Ship's corporal (Naut.), a petty officer who assists the
      master at arms in his various duties.
      [1913 Webster]