1. To sink, in the middle, by its weight or under applied pressure, below a horizontal line or plane; as, a line or cable supported by its ends sags, though tightly drawn; the floor of a room sags; hence, to lean, give way, or settle from a vertical position; as, a building may sag one way or another; a door sags on its hinges.
2. To lose firmness or elasticity; to sink; to droop; to flag; to bend; to yield, as the mind or spirits, under the pressure of care, trouble, doubt, or the like; to be unsettled or unbalanced. the mind I sway by, and the heart I bear, Shall never sag with doubt nor shake with fear. (Shak)
Origin: Akin to Sw. Sacka to settle, sink down, LG.sacken, D. Zakken. Cf. Sink.