It's like Hitler, only worse.

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.

To apply the feces from one's ass to your own or another persons upper lip.

A Shitler is when you give a Rimjob to your lover and they have a dirty ass so you get a brown patch on your top lip that looks like a Hitler mustache