Payment (whether or not in the form of a check) is en route or will be sent shortly. Often used as an excuse to avoid the pressure of creditors or someone expecting payment. Primarily heard in US. A: "Mr. Smith, your mortgage payment is now two months overdue." B: "The check's in the mail, I promise you!"A: "Can you lend me $40 until I get paid next week?" B: "Sure thing, the check's in the mail."