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1. Literally, to locate someone or something using one's sense of smell. Almost exclusively said of dogs. A noun or pronoun can be used between "sniff" and "out." We've sent a pack of dogs to sniff the fugitive out. The police use specially trained dogs to sniff out drugs and bombs.
2. By extension, to uncover, reveal, or expose someone or something through some form of investigation. A noun or pronoun can be used between "sniff" and "out." If the company is up to anything fishy, the auditor will sniff it out. Our lead investigative journalist sniffed out the politician leaking information to foreign intelligence agents.
sniff someone or something out
to locate someone or something by sniffing or as if by sniffing. The dog sniffed the intruder out and the police captured him. The dog sniffed out the mole in the lawn.
Uncover, as If there's anything to that rumor, Gladys will sniff it out. This expression alludes to an animal sniffing for prey. [First half of 1900s]
To perceive or detect someone or something by or as if by sniffing: The dogs sniffed out the trail through the snow. The detectives sniffed the plot out and arrested the criminals.