underhand

adjective

he was accused of employing underhand tactics:

deceitful, underhanded, dishonest, dishonorable, disreputable, unethical, unprincipled, immoral, unscrupulous, fraudulent, cheating, dubious, dirty, unfair, treacherous, duplicitous, double-dealing, below the belt, two-timing, two-faced, Janus-faced, unsporting, unsportsmanlike;

devious, calculating, artful, crafty, cunning, conniving, scheming, designing, sly, wily, guileful, tricky;

criminal, illegal, unlawful, nefarious;

secret, secretive, clandestine, surreptitious, sneaky, sneaking, furtive, covert, veiled, shrouded, cloak-and-dagger, hugger-mugger, hole-and-corner, hidden, back-alley, backstairs, under the table, conspiratorial;

North American snide, snidey;

informal crooked, shady, bent, lowdown, murky, fishy;

British, informal dodgy;

Australian, New Zealand, informal shonky;

South African, informal slim.

▷antonyms honest, fair, above board.