This interesting word came into English from French probably when Shakespeare was in his twenties, and it was avidly seized upon by several writers. It always had the general sense of "manner, style, fashion". Avoid the "clothing" sense in Shakespeare, for that did not evolve until a decade after his death. So, when Iago says to himself, of Cassio, "I'll ... Abuse him to the Moor in the rank garb" (Othello, II.i.297), he is talking about what he's going to say and not about how he's going to look when he says it. Gower describes the Welshman Fluellen as not speaking English "in the native garb" (Henry V, V.i.80). And Hamlet brings his conversation with Rosencrantz and Guildenstern to a close, saying: "Let me comply with you in this garb" (Hamlet, II.ii.372) - "let me observe the courtesies with you in this way", shaking hands with them.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin