The phrase "the check is in the mail" means that payment is forthcoming or will be sent shortly. The expression is often used as an excuse rather than an affirmative response.
The excuse is mostly used when a creditor begins to pressure a debtor for payment. The phrase is sometimes used ironically, with the undertone that the creditor is not going to receive payment at all. The phrase is primarily used in the United States.
The phrase originates from the once-common practice of customers paying bills by sending checks to companies by mail. In modern times, the popularity of checks has declined with the emergence of debit cards and online banking. However, the phrase survives and is used even when the payment is not in the form of a check.