underhand has 4 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is overhand.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
with hand brought forward and up from below shoulder level
“an underhand pitch”
marked by deception
“achieved success in business only by underhand methods”
Synonyms, antonyms and the broader and narrower relations for underhand come from Princeton University’s WordNet. Full sources and licences.