A large net dragged along the bottom of water to catch fish, or a systematic search or operation to catch criminals.
Compound of 'drag' and 'net.' The fishing meaning came first; by the 20th century, police used the term 'dragnet' for their sweeping operations to round up suspects.
The TV show 'Dragnet' from the 1950s made the police version so famous that most people today don't realize it originally meant an actual fishing net!
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