A term spawned from two previously immature and overused insults 'your mum' and 'your face'. Although these two retorts were originally used by the illiterate and mono-sylavic lesser-beings of british culture the combining of the two has only recently been adopted by somewhat higher brow citizens.
Although used gramatically in much the same fashion as both its parent phrases the use of 'Your mum's face' is generally accepted to be a satirical take on modern chav culture.
"Man this seat is comfy"
"your mum's face is comfy"
"Don't you think this ice cream is delicious"
"Your mum's face was delicious last night"