arm in arm
With one person's arm linked around another's; also, closely allied or intimate, as in Both couples walked arm in arm around the grounds of the estate, and This candidate is arm in arm with the party's liberal wing. The literal expression dates from the late 1300s, when Chaucer so used it: "They went arm in arm together into the garden" (Troilus and Cressida). The figurative usage dates from about 1600. Also see hand in hand.