What is Porridge Muncher?
1.
To go down on (felate) your mate's (friend's) girlfriend or recently ex girlfriend.
(in that he may have consumed some of your semen).
You went down on my ex?
You're a porridge muncher!
You munched my porridge, ah ha. Porridge muncher.
Or
He went down on your girl?
What a porridge muncher.