[1] : WordNet (r) 2.0

underweight
     adj : having unattractive thinness; "a child with skinny freckled
           legs"; "a long scrawny neck" syn: scraggy, scrawny,
            skinny, weedy
see also:
scraggy scrawny skinny weedy 

[2] : Moby Thesaurus II by Grady Ward, 1.0

75 moby thesaurus words for "underweight":
   avoirdupois, bantamweight, beef, beefiness, boniness, bony,
   deadweight, fatness, featherweight, flat, flat-chested, fleshless,
   fleshlessness, frailness, gangling, gangly, gaunt, gauntness,
   gawkiness, gawky, gravity, gross weight, haggardness, hatchet face,
   heaviness, heft, heftiness, lank, lankiness, lankness, lanky,
   lantern jaws, lean, lean-fleshed, lean-looking, leanness,
   lightweight, liveweight, meager, meagerness, neat weight, net,
   net weight, overbalance, overweight, paperiness, peakedness,
   ponderability, ponderosity, ponderousness, poorness, poundage,
   puniness, rawboned, scraggy, scrawniness, scrawny, skeletal,
   skin and bones, skinniness, skinny, slightness, spare, spareness,
   spidery, spindling, spindly, thin-bellied, thin-fleshed, tonnage,
   twigginess, twiggy, undersized, weight, weightiness

Underweight >> U Words
Underweight, definition of term: Underweight
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