"The coupon is the interest rate stated on a bond when it is issued. The coupon is typically paid semiannually. This is also referred to as the "coupon rate" or "coupon percent rate."
The coupon may have the interest rate written on it. But the coupon is not the interest rate or the interest for that matter. Bonds made payable to bearer had certificates of interest, which could be cut from the bond and presented for payment. These pieces of paper cut off from the bonds are the coupons. 
From 1822, "certificate of interest due on a bond"