Crossword Dictionary
underhand
underhandedly, underhand - r
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
Use the adjective underhanded to describe someone who plays dirty tricks, like a cheating poker player or a manipulative classmate.
It would be underhanded to spread gossip about your opponent during a student council election, just as it's underhanded for a business owner to pay herself more while laying off workers. If it's unfair and sneaky, it's underhanded — unless you're pitching a softball, in which case you're simply throwing it from below your shoulder instead of above.
etymology
mid-14c., "by secret means, stealthily, in a surreptitious manner," from under + hand (n.). Perhaps the notion is of the hand turned over (thus concealing what it holds). Compare Middle Dutch onderhanden "by degrees, slowly," Dutch onderhandsch "secret, private." The adjective is attested from 1540s. Old English under hand meant "in subjection, in (one's) control or power."