Bobby

google

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mid 19th century: pet form of Robert, given name of Sir Robert Peel, Sir Robert.

wiktionary

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From the given name of Sir Robert Peel, who established London’s police force. [1]

bobby (plural bobbies)

etymonline

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bobby (n.)

"London policeman," 1844, from the familiar diminutive form of the masc. proper name Robert, in reference to Mr. (later Sir) Robert Peel (1788-1850), Home Secretary who introduced the Metropolitan Police Act (10 Geo IV, c.44) of 1829. Compare peeler.