n. chiefly british slang, derogatory
1. one who has a face that has a similar appearance to the buttocks
2. one who is of such an annoying temperament that they are worthy of being compared to someone with a bum for a face
3. Someone with a bum for a face. And perhaps a face for a bum, although this is uncommon.
"Oh my god, you are such a bum face".
"Good golly, you're talking so much poo right now, one might call you a bum face"