sagging

from The Century Dictionary.

• noun That form of breakage in which the middle part sinks more than the extremities: opposed to hogging.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A manner of wearing pants or shorts below the waist, revealing some or all of the underwear.
• verb Present participle of sag.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective hanging down (as from exhaustion or weakness)