Emily Dickinson Lexicon

RAP, v.i. [Sax. hrepan, hreppan, to touch; repan, to touch, to seize, L. rapio; Sw. rappa; Dan. rapper, to snatch away, and rapper sig, to hasten; rap, a stroke, Sw. rapp; Fr. frapper, to strike. The primary sense of the root is to rush, to drive forward, to fall on, hence, both to strike and to seize. That the sense is to drive or rush forward, is evident from L. rapidus, rapid, from rapio. See Class Rb, No. 26, 27, 28, 29.]

To strike with a quick sharp blow; to knock; as, to rap on the door.