A coupon for half-off, pre-chewed gum. Or a two-for-one deal on excremental face cream. You know, a gross coupon.
In finance, the term refers to a payment due on a debt instrument based on pool of assets. The most common example is mortgage-backed securities.
A coupon here refers to payments made on debt securities, like bonds. They're called coupons because you used to have to tear off small paper coupons and present them for payment.
Since pooled securities like mortgage-backed securities include a bundle of individual loans, the interest rate is a blend of all the particular mortgages (or whatever) that are collected. So the coupon represents the gross amalgam.