underhand

with hand brought forward and up from below shoulder level; "an underhand pitch"; "an underhand stroke"

marked by deception; "achieved success in business only by underhand methods"

slyly 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

with the hand swung below shoulder level; "throwing a ball underarm"