quicksand (n.)

"movable, very loose sand bank in a sea, lake, or river," capable of swallowing heavy objects and sometimes dangerous to vessels or travelers," c. 1300, from Middle English quyk "living" (see quick (adj.)) + sond "sand" (see sand (n.)). Figurative use by 1590s. Old English had cwecesund, but this might have meant "lively strait of water."