underhand

Synonyms: clandestine concealed crafty crooked cunning deceptive devious dirty-dealing dishonest dishonorable double-crossing duplicitous fraudulent furtive guileful hush-hush indirect insidious oblique on the qt on the quiet secret secretive shady shifty slippery sly sneaking sneaky stealthy sub-rosa surreptitious treacherous tricky two-faced two-timing undercover underhanded under wraps unethical unfair unjust unscrupulous wily

Antonyms: aboveboard forthright frank honest