counter for helmets
Archaic counter used specifically for helmets (兜) and similar headgear. Not productive in modern Japanese; encountered only in historical or literary contexts.
In old war tales, helmets are sometimes counted as 一刎 (one helmet).
behead; decapitate; cut off someone's head
The kanji 刎 originally means 'to cut off' or 'behead', and its use as a counter for helmets is likely derived from the shape or the act of cutting off a helmeted head. The exact derivation is uncertain.