When one takes a nap while sitting on the toilet. The shnap generally occurs at the work place and is the result of over eating, a bad hangover, boredom or a combination of the three. In order for one to take a shnap, he/she does not need to actually take a shit. The purpose of the shnap is that toilet may be the only place an employee can nap during the day.
I was so hungover on Friday that I went and took a shnap around lunch time. When I woke up 15min later I tried to stand up, but my legs were completely asleep, so I sat down and took another shnap.