E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A coward. Menage derives it from the Italian poltro, a bed, because cowards feign themselves sick a-bed in times of war. Saumaise says it means maimed of the thumb, because in times of conscription those who had no stomach for the field disqualified themselves by cutting off their right thumb. More probably a poltroon is a hawk that will not or cannot fly at game. (See above.)