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.
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