WordNet (r) 3.0 (2006):

underweight
    adj 1: being very thin; "a child with skinny freckled legs"; "a
           long scrawny neck" [syn: scraggy, boney, scrawny,
           skinny, underweight, weedy]

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