Middle English quaken, from Old English cwacian "quake (of the earth), tremble, shudder (of persons, from cold, emotion, fear, fever, etc.), chatter (of teeth)," related to cweccan "to shake, swing, move, vibrate," words of unknown origin with no certain cognates outside English. Perhaps somehow imitative (compare quag, quaver, quiver (v.), Middle English quaven "tremble, shake, palpitate," c. 1200). Related: Quaked; quaking. In Middle English formerly also with strong past-participle form quoke. The North American quaking aspen is so called by 1822.
early 14c., "a trembling in fear," from quake (v.). Rare except in combinations, and now usually as a shortening of earthquake, in which use it is attested from 1640s. Old English had the verbal noun cwacung "shaking, trembling." Also compare Middle English quavinge of erþe "an earthquake" (14c.), earthquave (n.), early 15c.