Hallux, plural halluces.
1. The innermost or first digit on the hind foot of certain mammals. In humans the hallux is the big toe.
2. The equivalent digit of a bird, reptile or amphibian. In birds it is often directed backward.

The hallux corresponds to the pollex, or thumb, of the fore limb.
According to the OED the first recorded use was in an anatomy book of 1831.
The suggestion is that it is a medieval Latin blend of allus, hallus (thumb) and hallex (big toe).