underhand (adv.)youdaoicibaDictYouDict[underhand 词源字典]
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."[underhand etymology, underhand origin, 英语词源]