nicotash
The foul smelling sticky brownish black gunk left at the bottom of the ashtray after emptying it. Sometimes coated with a layer of fine gray ash, if you wipe it off with your hand it will stick and then stink as foul as the ashtray. If left to accumulate long enough it is quite difficult to scrape off and may harden.
If most smokers had any sense they'd see all that nicotash in the bottom of their ashtrays and realize that is what the bottom of their lungs looks and smells like, too.