This is taken from the advert for Dolmio pasta sauce, featuring a happy Italian family who all end up with mucky red faces after eating spaghetti bolognese with too much gusto. The slang term describes the state of a man's face after performing oral sex on a woman during her "time of the month".
So he's gone to the bathroom for a slash, and discovers to his horror that he's wearing the Dolmio grin...
by Darwin March 31, 2005