Well, the results produced no such thing, but a reddit thread titled [Serious] Why are the CH-53 Stallions nicknamed "Shitters"? was the first result.
Among the answers:
I believe they were dubbed Shitters by grunts because the 53 lands, and shits out a platoon of Marines.
My platoon sergeant in the Army was a former Marine -46 guy, he said they were called "shitters" because from a distance you can see the smoke they're shitting out.