"Ya mum" is used as an insult because most people would take offense at a slur directed at their mother.
The term ya mum is a slang for an insult your mother. It is usually used by street gangsters and is considered "gangsta" (as one can see the ending of both words is the same ends in a, even though it is not correct).
Ya mum! Ya mum! Ya mum! Ya mum! Ya mum! Ya mum! Ya mum! Ya mum! Ya mum! Ya mum!
ya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mumya mum ya mum
ya mum ya mum ya mum ya mum ya mum
Ya Mum Ya Mum Ya Mum Ya Mum Ya Mum Ya Mum
its ya mum
ya mum
depends what you are saying 'your mum' about.
ya mum
ya mum ya mum
ya mum mum ya hwehehwhe
there is not a organism in the whole universe as heavy as ya mum