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.
The play tells the story of a childless woman living in rural Spain. Her desperate desire for motherhood becomes an obsession that eventually drives her to commit a horrific crime. This desperation is produced by the social norms of her culture, and the work functions as a critique of those mores.
Yerma is one of the three tragic plays which form Lorca's famous 'Rural trilogy'. The others being Bodas de sangre ('Blood Wedding') and La Casa de Bernarda Alba ('The House of Bernarda Alba'). The trilogy similarly emphasise the submissive position of women who desire freedom in a traditional society which denies them social or sexual equality.
Yerma deals with the themes of isolation, passion and frustration. Social conventions of the period also play a large part in the play's plot.
Yerma accounts for the status of the main character in the play, unable to produce any offspring.
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."
"Ay yer ma!
(Listen to my audio definition.. it's of how a steek would pronounce it)