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.