It is the scat equivalent of "snowballing" (which would normally happen with semen). It means swapping feces back and forth between two people's mouths.
Simply means "inter-racial sex". Can be used as "mud-larking" (hyphenated) or "mudlarking". It is non-derogatory and non-condemnatory. It appliesequally to either female or male pursuits. Often it is used when referring to "exploratory" experiences.
"Well, I think she was just in for the mudlark"...