"expanse of dark igneous rock," 1794, from Swedish trapp (Torbern Bergman, 1766), from trappa "stair," related to Middle Low German trappe "staircase" (see trap, n.). So called from the step-like appearance of the rock.

"drums, cymbals, bells, etc.," 1925, from earlier trap drummer (1903) "street musician who plays a drum and several other instruments at once," perhaps from traps "belongings" (1813), shortened form of trappings.