The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.]
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            The mind I sway by, and the heart I bear,
            Shall never sag with doubt nor shake with fear.
                                                  --Shak.
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   3. To loiter in walking; to idle along; to drag or droop
      heavily.
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   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.
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The Collaborative International Dictionary of English v.0.48:

Sag \Sag\, v. t.
   To cause to bend or give way; to load.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sag \Sag\, n.
   State of sinking or bending; sagging.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sag
    n 1: a shape that sags; "there was a sag in the chair seat"
         [syn: sag, droop]
    v 1: droop, sink, or settle from or as if from pressure or loss
         of tautness [syn: sag, droop, swag, flag]
    2: cause to sag; "The children sagged their bottoms down even
       more comfortably" [syn: sag, sag down]