The word duck is an abbreviation of the term 'Silent Duck' which refers to the shape made by the hand during fisting, or initiation into fisting. Silent Duck
And so the term cheeky duck refers to slyly slipping a silent duck in during sex, aimed to catch the receiver off guard.
Slang for smegma: the secretion of a sebaceous gland
specifically : the cheesy sebaceous matter that collects between the glans penis and the foreskin or around the clitoris and labia minora
After going three days without a bath, Charlie had a buildup of duck cheese on the head of his penis.