To breathe air quickly through your nose, usually to smell something, or to show disapproval.
From Middle English 'sniffen,' likely imitative in origin, mimicking the sound of breathing through the nose. The word has been used since at least the 1500s to describe both smelling and expressing disdain.
The word 'sniff' is onomatopoeia—it literally sounds like the action it describes! Interestingly, we use the same physical action to both appreciate a pizza's aroma and to show we think something is beneath us, which says something about how closely linked smell and emotion are in human brains.
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