Source : Webster's Revised Unabridged Dictionary (1913)

Sag \Sag\ (s[a^]g), v. i. [imp. & p. p. {Sagged}; p. pr. & vb.
   n. {Sagging}.] [Akin to Sw. sacka to settle, sink down, LG.
   sacken, D. zakken. Cf. {Sink}, v. i.]
   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. Fig.: 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. [R.]

            The mind I sway by, and the heart I bear, Shall
            never sag with doubt nor shake with fear. --Shak.

   3. To loiter in walking; to idle along; to drag or droop
      heavily.

   {To sag to leeward} (Naut.), to make much leeway by reason of
      the wind, sea, or current; to drift to leeward; -- said of
      a vessel. --Totten.

Sagging \Sag"ging\, n.
   A bending or sinking between the ends of a thing, in
   consequence of its own, or an imposed, weight; an arching
   downward in the middle, as of a ship after straining. Cf.
   {Hogging}.

Source : WordNet®

sagging
     adj : hanging down (as from exhaustion or weakness) [syn: {drooping},
            {droopy}]

sag
     v 1: droop, sink, or settle from or as if from pressure or loss
          of tautness [syn: {droop}, {swag}, {flag}]
     2: cause to sag; "The children sagged their bottoms down even
        more comfortably" [syn: {sag down}]
     [also: {sagging}, {sagged}]

sag
     n : a shape that sags; "there was a sag in the chair seat" [syn:
          {droop}]
     [also: {sagging}, {sagged}]

sagging
     See {sag}