The innermost digit of the hind or lower limb; (in humans) the big toe.
'Hallux' is from the Latin word hallex, great toe. Similarly, 'pollex' is defined as the innermost digit of a forelimb and derives from the Latin pollex, thumb. The adjective forms are 'hallucal' and 'pollical', respectively.
Both words first appeared in English in the early 1800s. The earliest English references I've seen so far are in an 1817 dictionary (on Google books).