Similar in process to a Dirty sanchez. There are 2 times this can be done:
1) When a man, after anal sex, puts his finger inside a girl (or guy's) ass and takes alot of shit on his finger and wipes the crap above the his partner's lip, which resembles a hitler-like moustache.
2) A sleepover prank. When someone is sleeping wipe shit on above their lip.
The only diffrence between a shitler and a dirty sanchez is that a shitler is thicker and has more crap then a dirty sanchez.
1) Yo you gave that guy at the sleepover a shitler?
2) Yeah but i didnt bang him up the ass!!