Scheiser is a misspelling and mispronunciation of the german word scheiß or scheiße, which is commonly used improperly in American English. In standard German scheiße is used the same way Americans use fuck.
Scheisse (the English spelling) is also used as a prefix to describe something that is either unpleasant, disagreeable, or otherwise in some way considered to be bad or wrong.
Scheiße! Ich habe mich verlaufen!
Fuck! I'm lost!
Scheiße! Ich bin in die Tur meine Finger zuknallten!
Fuck, I slammed my finger in the door!
Ich finde Rauchen total Scheiss.
I think smoking is utterly dumb
Scheiser, Ich vergesse, das ich muss Scheisse sagen!
Scheiser, I forgot I am supposed to say Scheisse!
The German word for shit.
SCHEISER!! the deadline is today