An oyster pirate was a term for a person who, during the 1880s in the Oakland, California region of the planet Earth, raided the oyster beds of monopolistic oyster farmers at night and sold their take in the Oakland markets in the morning.
In 1893, a young Jack London mentioned to Data that among his wide spectrum of previous occupations he used to be an oyster pirate. (TNG: "Time's Arrow")
"Oyster pirate" is a term rarely if ever encountered outside of accounts of the life of Jack London. It is often mentioned without any explanation as if everyone knew the meaning of the term. London described oyster piracy in his autobiographical memoirs John Barleycorn.