A euphemism for ]pornography] suggesting quality periodicals and films for the discerning gentleman form the Victorian/Edwardian period (1880-1920). Almost certainly deriving from the euphemism for semen or ejaculate.
I shall retire to my man cave with my latest delivery of high quality gentlemen's relish and have a damned good wank.
Noun: Spunk, jism, come baby gravy. There is actually a brand of anchovy paste in Britain called The Gentleman's Relish - Patum Peperium. Coincidentally, it's rather salty.
Do you like the taste of Gentleman's Relish, babe?
Don't know, never tried it.
Open wide. What's your name by the way?
"Gentleman's Relish" (n), syn "duck butter". A smelly, warm, wet film comprised of sweat deposits, dead skin, water, pubic hairs and similar miscellaneous detritus that collects between the male scrotum and thigh.