Source: Collector's / martial artist's jargon
Cantonese pronunciation for bāzhǎndāo (八斬刀) or "eight cutting knives". An alternative name for the húdiédāo (蝴蝶刀).
Various different transliterations are used, among which baat jaam do, bat jaam dao, etc.
Bat jam do is the most commonly used term for them in southern Chinese martial arts circles.
See: húdiédāo (蝴蝶刀)