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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 (cf. Dutch zakken, Swedish sacka, Danish sakke). Of body parts from 1560s; of clothes from 1590s. Related: Sagged; sagging.
1580s, in nautical use, from sag (v.). From 1727 of landforms; 1861 of wires, cables, etc.