buffalo fart
The stale, sour smoke left in a bong chamber from an unfinished hit.
A stoner faux pas, passing a bong containing a buffalo fart is akin to farting in a full elevator. 

Also used in the southern US to describe the rank smell of a burning marijuana seed. Failing to remove all the seeds is also a faux pas due to this stink.
"Dude, finish your hit! You handed me a buffalo fart!"
"Dude, clean your weed next time, that buffalo fart reeks!"