1a downward curve or bulge in a structure caused by weakness or excessive weight or pressure:a sag in the middle necessitated a third set of wheels
[mass noun] Geometry the amount of a sag, measured as the perpendicular distance from the middle of the curve to the straight line between the two supporting points.
2a decline, especially a temporary one.
1sink, subside, or bulge downwards under weight or pressure or through lack of strength:she let her head sag lower and lowerthe bed sagged in the middle (as adjective sagging)sagging shelves bearing rusty paint tins
hang down loosely or unevenly:stockings which sagged at the knees
2decline to a lower level, usually temporarily:exports are forging ahead while home sales sag