bulldog clip
A metal clip with two flat handles that are squeezed to open the jaws, often used for holding thick stacks of paper. The name comes from the round, eye-like shape of the handles.
The documents were too thick for a stapler, so I fastened them with a bulldog clip.
Compound of 目玉 (eyeball) and クリップ (clip), referring to the round, eye-like shape of the clip's handles.