The story
Earliest use in weapon (and later, agricultural) sense, bill (“a bladed pike (obsolete)”) + hook; other senses formed anew from various meanings of bill.
Etymology from Wiktionary, CC BY-SA 4.0.
billhook
Today it means: A medieval polearm, fitted to a long handle, sometimes with an L-shaped tine or a spike protruding from the side or the end of the blade for tackling the opponent; a bill. Full definition →