Definition of twibill:
(n) :
(carpentry) A two-edged tool used in gate-type hurdle-making for cutting out mortises, with a flat chisel and a mortise chisel or hook, similar to the much larger French carpenter's tool, the besaiguë (or bisaiguë).
(n) :
(dialectal, British) A mattock with one blade like an axe and the other like an adze.
(n) :
(dialectal, England) A reaping hook, especially for cutting beans and peas.
(n) :
(obsolete) A double-bladed halberd or battle-axe.