When you are among a large group of people (Usually people you don't know, like at school or a train station or a store) and suddenly the air becomes occupied by a nasty odour.
The act of "smell discrimination" comes into play when you decide by the people who surround you and their physical appearance/demeanour, who it is that created the disgusting smell.
(In a store)

Guy 1: "Dude, something smells like rotten eggs. I think it was that fat hairy guy over there" *Points*

Guy 2: "That's smell discrimination! It could have just as easily been that hot girl in the pink tank top!"
by Johnny Hay Hoo November 3, 2008
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