The term 'beat rapidly' can be summarized by two crossword solutions: 'PALPITATE' (9 letters) and 'THROB' (5 letters). PALPITATE is a verb meaning to beat rapidly and rhythmically, usually in reference to the heart. It is derived from the Latin word 'palpitare, ' which means to flutter or quiver. THROB is also a verb meaning to beat rapidly and rhythmically, usually in reference to a pulse or heartbeat. It is derived from the Old English word 'throbban, ' which means to beat or pulsate.