late 14c., possibly from a Scandinavian source related to Old Norse sokkva "to sink," or from Middle Low German sacken "to settle, sink" (as dregs in wine), from denasalized derivative of Proto-Germanic base *senkwanan "to sink" (see sink (v.)). A general North Sea Germanic word (compare Dutch zakken, Swedish sacka, Danish sakke). Of body parts from 1560s; of clothes from 1590s. Related: Sagged; sagging.