The story
From beartrap + gate. From being a gate reminiscent of the operation of a beartrap.
Etymology from Wiktionary, CC BY-SA 4.0.
beartrap gate
Today it means: A waterflow/impoundment hydraulically-operated rising double-leaf floodgate used for weirs and dam spillways, and canals, to control waterflow across the top of the gate in the channel. Full definition →