by return of post prepositional phrase2 •(UK) Replying in the next available mail delivery.In mercantile transactions an offer must as a rule be accepted by return of post, or at any rate by a letter posted on the day on which the offer was received. — 1909 •(UK, figurative) Replying immediately or as soon as possible.Osborn replied almost by return of post that he agreed with my suggestions if we could get together a competent group to do the work. — 1987
