A physical or (more usually) conceptual or virtual token for the later provision of oral sex, given by a somewhat cheap person, to someone (usually a lover or spouse) who has just provided goods or services that cannot be
immediately (physically or financially)
reciprocated.
By constant use the term 'coupon' itself comes to mean the thing for which it is a token, namely, oral sex.
Believed to be derived jointly from the prevalence of coupons in late post-industrial/
capitalist society (largely exchangeable for low-quality goos - see coupoff) and from the Scots/Irish/Gaelic 'coupon' meaning face - coupon/face then suggesting head or oral.