late 14c., saggen, "hang down unevenly," also in Middle English "sink, be mired, sink down," 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 by 1560s; of clothes by 1590s. Of other objects, "to droop, especially in the middle, as from weight or pressure" is by 1777. Related: Sagged; sagging.
"a bending or drooping," 1580s, in nautical use, "movement to leeward," from sag (v.). From 1727 in American English in reference to landforms having a sunken look. By 1861 in reference to droop from slackness in wires, cables, etc.
updated on November 06, 2021