To do s.t. with a snapping or clipping action (as when biting one's fingernails or a thin string into two pieces, snapping a briefcase shut, a clam snapping shut, clipping a bush, stapling papers together, buttoning a shirt that has snap buttons, etc.).
Why are you biting (lit. clipping with your teeth) your fingernails?
Snap your briefcase shut.
That giant clam snapped shut.