underhand (adv.)

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."

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Definitions of underhand from WordNet
underhand (adv.)
slyly and secretly; "Mean revenge, committed underhand"- John Donne; "oldline aristocratic diplomats underhandedly undermined the align Germany with the Western democracies"- C.G.Bowers;
Synonyms: underhandedly
underhand (adv.)
with the hand swung below shoulder level;
Synonyms: underarm
underhand (adj.)
with hand brought forward and up from below shoulder level;
an underhand pitch
an underhand stroke
Synonyms: underhanded / underarm
underhand (adj.)
marked by deception;
achieved success in business only by underhand methods
Synonyms: sneaky / underhanded