Underweight rhymes with:
counterweight, featherweight, paperweight, welterweight, actuate, antiquate, braithwaite, counterweight, cowperthwaite, deadweight, featherweight, fluctuate, garthwaite, goldthwaite, heavyweight, hundredweight, menstruate, middleweight, paperweight, perpetuate, welterweight
Underweight sounds like:
unaddressed, unauthorized, undercoat, undercooked, undercut, underside, undersized, understate, understated, understood, understudy, underused, underweighted, undressed, uninterested