short dagger; nine-sun-five-bu dagger
Archaic term for a dagger with a blade length of about 9 sun 5 bu (roughly 28.8 cm). Used in historical or literary contexts.
In period dramas, there are scenes where a samurai conceals a nine-sun-five-bu dagger in his breast fold.
短刀 is the general modern term for a short sword or dagger, while 九寸五分 refers specifically to a historical dagger of a particular length.
From the blade length of 9 sun 5 bu (九寸五分), a traditional Japanese measurement. The exact historical origin of the term is uncertain.