An insulting phrase used in glasgow, quite possibly in other scottish cities too.
Pump: verb. To have sexual intercourse.
Yer Maw: noun. A slang way of referring to another person's mother.
Pump Yer Maw: To have sexual intercourse with another person's mother.
"Awwww mate, ah pumped yer maw last nite"
"Awwww mate, am pumpin yer maw right now"
(possibly talking on the phone)
"Awwww mate, am gonnae pump yer maw the nite"