butt of a gun
Refers specifically to the rear end of a firearm's stock, the part that rests against the shoulder.
猟師は台尻を肩に当てて狙いを定めた。
The hunter placed the butt of the gun against his shoulder and took aim.
Compound of 台 (stand, base) and 尻 (buttocks, rear). The exact historical derivation is uncertain, but the combination transparently describes the rear part of a gunstock.