A sluncher is something that you eat too late to be a lunch, too soon to be a dinner and too large to be a snack. A sluncher is the single meal you have in a day, neither at lunch nor dinner time.
Similarly as a brunch is a breakfast + lunch, a sluncher is considered a snack + lunch + dinner.
I had a sluncher = I didn't have lunch nor dinner but instead I ate a complete meal at a time in between the two.
Slanched is to show that you are sitting in a manner that is not polite and is to show you're notsitting up but rather in a position to show that you're in a bad mood