When a great coup is planned, then it is certain from the beginning: someone must, during the (mostly) criminal act Keep a lookoutso that no one can still ruin the possibly meticulously thought-out plan.
Once again, appearances are deceptive, because this expression has absolutely nothing to do with oil or fat.
Once again, we owe this idiom to the appropriately revered crook language. So the expression was not invented by the local crooks, no, but at least carried by them into the German language. Its linguistic origin finds the idiom namely in the Yiddish, where the word "schimro" guard, guarding means. In German, it was then simply adapted to the already existing word "Schmiere" - et voilà: Schmiere stehen.