Contrary to popular beleif the term 'yer ma' doesn't actually originate in Scotland. It has been used by many 'spides' or 'steeks' in the beautiful county of Northern Ireland for many many years. As stated above, it is used in a vulgar way when in an argument, when the spide/steek can't think of anything else to say.
Generally it is used to imply that that the spide/steek has had sexual relations with one's mother... but it is usually not the case, since most spides/steeks couldn't get laid if they tried.

spide slag for when they can't think of anything else to say better than it

"yer ma" is a generic response when you can't think of anything else to say. Mostly used in Scotland, and by yer ma (see...)

"Yer ma" is used by people in Northern Ireland or norn iron as an insult to someone.

Yer ma is an insult often used by "mokes", "steeks", "spides" and "millies".

Basically it means your mother, often return firing an insult, or saying I had (slept) with your mother.

Return fire with something like "You did mate.. I would seriously consider visiting a doctor about that, coulda caught something" "What would you rate her? 8 out of 10? She's good isn't she?" "My "You dug her up from the ground you sick bastard?!" "She's dead" (I hate this one..) "She's had a lot of people you know."

All of these definition's say its either from Scotland or Northern Ireland but thats bullshit its from Dublin.
"Yer Ma" Is a phrase used as a comeback to ANYTHING.
It can be a part of a long insult(Ex.1) or just used as itself.(Ex. 2)

A crappy inslut that has been said to me 100's of thimes when I clamp chavs and they have nothing to say. Chavs are the scum of Scotland, neigh...the WORLD!