1. a bizzarre sexual practice involving smearing shit on your partner's top lip after anal sex, see dirty sanchez
2. same as above, but done as a practical joke, usually whilst someone is sleeping/passed out etc. turd applied may or may not be human. the person wakes up to a most fragrant aroma.
slimy perv: fancy a shitlip tonight?
your daughter: okay!!!
Matt: haha look we just totally shitlipped joey with that dog crap we found outside!
joey: what..uh..how long was i out for? and uh, whats that smell?