Open English Wordnet
Adjectives
(a)underhand, underhanded, underarmwith hand brought forward and up from below shoulder level“an underhand pitch”, “an underhand stroke”
(s)sneaky, underhand, underhandedmarked by deception“achieved success in business only by underhand methods”
Adverbs
(r)underhandedly, underhandslyly and secretly“Mean revenge, committed underhand” (John Donne), “oldline aristocratic diplomats underhandedly undermined the attempt … to align Germany with the Western democracies” (C.G.Bowers)