This meaning is used for someone who want it to put in your ass, people who most commonly use this term really want it in the ass, the mum part is a way of saying the you have sexual relations with someone mother.
Someone: What are we going to do
You: I'll Put it your ass, like i'll put in your mum
Someone: OMG yess baby
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. Penguin Books,1992. p. 38)