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."
Entries related to underhand
Old English under (prep.) "beneath, among, before, in the presence of, in subjection to, under the rule of, by means of," also, as an adverb, "beneath, below, underneath," expressing position with reference to that which is above, from Proto-Germanic *under- (source also of Old Frisian under, Dutch onder, Old High German untar, German unter, Old Norse undir, Gothic undar), from PIE *ndher- "under" (source also of Sanskrit adhah "below;" Avestan athara- "lower;" Latin infernus "lower," infra "below").
Productive as a prefix in Old English, as in German and Scandinavian (often forming words modeled on Latin ones in sub-). Notion of "inferior in rank, position, etc." was present in Old English. With reference to standards, "less than in age, price, value," etc., late 14c. As an adjective, "lower in position; lower in rank or degree" from 13c. Also used in Old English as a preposition meaning "between, among," as still in under these circumstances, etc. (though this may be an entirely separate root; see understand).
Under the weather "indisposed" is from 1810. Under the table is from 1913 in the sense of "very drunk," 1940s in sense of "illegal" (under-board "dishonest" is from c. 1600). To keep something under (one's) hat "secret" is from 1885; to have something under (one's) nose "in plain sight" is from 1540s; to speak under (one's) breath "in a low voice" is attested from 1832.
Old English hond, hand "the human hand;" also "side, part, direction" (in defining position, to either right or left); also "power, control, possession" (on the notion of the hand's grip or hold), from Proto-Germanic *handuz (source also of Old Saxon, Old Frisian, Dutch, German hand, Old Norse hönd, Gothic handus), which is of uncertain origin.
The original Old English plural handa was superseded in Middle English by handen, later hands. Indo-European "hand" words tend to be from roots meaning "seize, take, collect" or are extended from words originally meaning only a part of the hand (such as Irish lam, Welsh llaw, cognate with Latin palma and originally meaning "palm of the hand"). One ancient root (*man- (2)), represented by Latin manus is the source of Old English mund "hand," but more usually meaning "protection, guardianship; a protector, guardian."
Meaning "manual worker, person who does something with his hands" is from 1580s, hence "hired workman" (1630s) and "sailor in a ship's crew" (1660s). Meaning "agency, part in doing something" is from 1590s. Clock and watch sense is from 1570s. Meaning "round of applause" is from 1838. The linear measure of 4 inches (originally 3) is from 1560s, now used only in giving the height of horses. The meaning "playing cards held in one player's hand" is from 1620s; that of "a round at a card game" is from 1620s. Meaning "handwriting" is from late 14c.; also "one's style of penmanship" (early 15c.). The word in reference to the various uses of hands in making a pledge is by c. 1200; specifically "one's pledge of marriage" by late 14c.
First hand, second hand, etc. (mid-15c.) are from the notion of something being passed from hand to hand. At hand is from c. 1200 as "near in time," c. 1300 as "within reach." Out of hand (1590s) is opposite of in hand "under control" (c. 1200). Adverbial phrase hand-over-fist (1803) is nautical, suggestive of hauling or climbing by passing the hands one before the other alternately.
Phrase on the one hand ... on the other hand is recorded from 1630s, a figurative use of the physical sense of hand in reference to position on one side or the other side of the body (as in the lefthand side), which goes back to Old English Hands up! as a command from a policeman, robber, etc., is from 1863, from the image of holding up one's hands as a token of submission or non-resistance. Hand-to-hand "in close contact," of fighting, is from c. 1400. Hand-to-mouth "said of a person who spends his money as fast as he gets it, who earns just enough to live on from day to day" [Bartlett] is from c. 1500. Hand-in-hand attested from c. 1500 as "with hands clasped;" figurative sense of "concurrently" recorded from 1570s.