Synonyms for Underhand (same or very similar meaning)
WordNet sense 1 (with hand brought forward and up from below shoulder level):
underarm, underhandedWordNet sense 2 (marked by deception):
sneakyWordNet sense 3 (not straight; dishonest or immoral or evasive):
corrupt, crooked, crookedWordNet sense 4 (slyly and secretly):
underhandedlyFrom the ODE community, based on WordNetadd/edit