the hand opposed to the left hand, late Old English rihthand; see right (adj.2) + hand (n.). So called as the one normally the stronger of the two. Applied to the right side generally by c. 1200. As a symbol of friendship or alliance, by 1590s. Figurative for "indispensable helper, person of use or importance," 1520s (right-hand man is attested by 1660s). Right-handed "having the right hand more useful than the left" is attested from late 14c.; as an adjective from c. 1700. Right-hander, of persons, "one who uses the right hand more skillfully than the left" is by 1885.
in reference to combat or competition, "hand to hand," 1970s, Spanish, from mano "hand," from Latin manus "hand" (from PIE root *man- (2) "hand").
to win something hands down (1855) is from horse racing, from a jockey's gesture of letting the reins go loose in an easy victory.
The Two Thousand Guinea Stakes was not the best contested one that it has been our fortune to assist at. ... [T]hey were won by Meteor, with Scott for his rider; who went by the post with his hands down, the easiest of all easy half-lengths. Wiseacre certainly did the best in his power to spoil his position, and Misdeal was at one time a little vexatious. [The Sportsman, report from April 26, 1840]
Ancient Greek had akoniti "without a struggle, easily," from akonitos (adj.), literally "without dust," specifically "without the dust of the arena."
Latin word once used in various phrases in English, often in legal language, where it means "the condition of something, the matter in hand or point at issue;" literally "thing" (see re). For example res ipsa loquitur "the thing speaks for itself;" res judicata "a point decided by competent authority."