The origin of the Royal Navy’s nickname is obscure. Some sources, including the Admiralty Manual of Seamanship, say that it derives from one Andrew Miller, a zealous officer of the Impress Service during the French Revolutionary and Napoleonic Wars, who ‘recruited’ so many men to His Majesty's ships that the navy was said to belong to him.
Earlier (19th century) sources variously suggest that Andrew was a slang name for a man o' war; that Andrew Miller was a supplier of provisions to the Royal Navy, with such an apparent monopoly that he was said to own the Navy; or that the Press Service officer was named Andrew Walker. The most reliable list of Royal Navy officers does not mention either an Andrew Miller or an Andrew Walker.
What is certain however, is that the Royal Navy continues to be known as the Andrew, and that there is no absolutely conclusive answer to the derivation of the nickname.