keep someone at arm's length

Example: “Since the argument, she has kept her cousin at arm's length.”

Origin: An earlier version, "at arm's end," is documented from the 1600s, including in Shakespeare's As You Like It (c. 1599): "hold death awhile at arm's end." By the 1700s, "end" had shifted to the now-standard "length," but the core image stayed constant — the literal distance a fully extended arm creates between two people is the shortest reach at which contact is still just barely possible, making it a natural, physically intuitive measure for deliberate but not total emotional or social distance. Various specific origins have been proposed beyond the plain gesture itself, including formal social etiquette governing how closely people of different rank were expected to stand, and even boxing, where a weaker fighter might physically hold a stronger opponent at that same distance to avoid a close, dangerous exchange — though none of these more specific stories is confirmed over the plain physical gesture as the actual source.