The muddy hallway is a term used to describe the sensation felt when engaging an extremely slutty girl in anal sex.
Multiple poundings from behind have left her back door gaping open. The tightness one would hope to feel is no longer available, resulting in a sensation similar to trying to engage a hallway in sexual intercourse.
Dude, I totally experienced a muddy hallway when I nailed Kim in her backdoor; she has one loose caboose. Something tells me I wasn't the first (or 50th) guy to do that.