I may be going out on a limb, here, but I've always regarded a leg, like an arm, as a limb both are attached to a girdle and, while the armbone is attached to the pectoral girdle, the legbone is attached to the pelvic girdle. The tailbone, however isn't a limb and is attached to the spine bone.

I hope I've demonstrated I'm disinterested but not uninterested.

Cheers, Rowan