A phrase that has been ironically quoted by many vtubers after a Twitter user posted screenshots of an alleged vtuber asking for free art saying "I'm a vtuber and you'll regret this" after the artist refused.
The only real comeback 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.