Also known as: 鉈鎌
A billhook is a tool for cutting branches and grape vines, usually with a curved, concave sickle-like blade. The length of the blade and handle varies.
The weapon version of the billhook is a Polearm typically with a pointed spearhead and back spike or hook in addition to the curved blade, similar to a Halberd.
Don't mix this up with the similar Hashitsuki Nata and Nata (Tool), which usually have a flat side and for which the "nose" is perpendicular to the blade, while the end of billhooks curves downward.