Refers to your anus and is generally used when talking about a promiscuous woman. The word is often used when describing a sexual encounter or sexual event that happened. The word is never said around women and usually used with "Bros".
"Yeah, so she let me put it in her mud pipe
"Dude I need to change that sheets ever time we do it in the mud pipe
"Yo you think thats good? Wait till you try her mud pipe