One of the mysteries of the English language finally explained.
A kind of mechanical grab for obtaining a sample of the bed of a body of water, consisting of two jaws semicircular in vertical cross-section and hinged along the top, arranged so as to close automatically on contact with the bottom.
1920s. From the name of Carl Georg Johan Petersen, Danish marine biologist, who invented it + grab.
Petersen grab/ˈpiːtəs(ə)n ˌɡrab/
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