When someone smells something gross and says, "ugh, nasty! Do you smell that?" and you breathe deeply to take in the smell and confirm said nastiness, you have just performed a courtesy sniff.
George: Gross, I think the cat peed on my rug. Paul, will you come give it a courtesy sniff?

Paul: (bends face towards rug and breathes in) Dude, yeah, cat totally peed on your rug. Sorry, bro.
by pseudonym jr August 26, 2011
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The act of sniffing a fart after having previously been warned of the fart.
Mike: Damn, I farted.
James: *sniff* damn that smells like ass. courtesy sniff like a mutha fucka
by mike9 March 7, 2007
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