I know what you are talking about.
A couple of weeks ago I had the misfortune to stand beside a gentleman in a store who smelled SO bad it's difficult to put into words. And as you say, it wasn't a typical I-haven't-showered-for-four-days smell, either. He LITERALLY smelled like poo had been fermenting between his ass cheeks for several weeks. Un-fuckin'-real.
I actually gagged.