On a long haired dog, cat, or sometimes, an unwashed co-worker, a combination of a loose bowel movement stuck in the hair around the butt. This mass has been sat upon, flattening it to a waffle-like appearance. In a co-worker, the presence of a butt waffle is indicated by a strong smell of "old shit".
"Hold still, you fucking cocksucker. If we don't get this butt waffle off yer ass, you'll have to stay outside for a week." (Generally screamed at a dog or cat, Not a co-worker.)
A bumwaffle is a usless person who acts like they know EVERYTHING!!!! Bumwaffle people also like crumpets.
Girl: Did you see Tiffany talking to Brian about hoovers?
Girl 2 : Oh my god she was such a bumwaffle!
Girl: I know she told me she hates hoovers!
Girl 2 : She has no boobs also!