The only realcomeback to ya mum that isn't whining about how infantile your insulter is. We can only assume what is meant by 'this'.
Scenario 1-boring, slightly civilised
Tony: What are you doing tonight?
Gordon: ya mum!
Tony: Now do we really have to resort to this childishness?
Gordon: Does ya mum have to resort to childishness?
Scenario 2- Childish, good
Tony: What are you doing tonight?
Gordon: ya mum!
Tony: keep my mum out of this and I'll keep this out of ya mum
Gordon: Yes, master.
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)