Why a Royal Navy first lieutenant is called Jimmy the One

Why were Royal Navy first officers called Jimmies or Jimmy the One?

The nickname combines the first lieutenant's position as Number One with an older word for neatness, reflecting responsibility for the ship's order and appearance.

The first lieutenant commonly served as the warship's executive officer and was also known as Number One. The nickname is connected to the older word 'jeminy,' meaning neatness or spruceness. Responsibility for the ship's order and appearance made the first lieutenant 'Jeminy the First.' Spoken usage then contracted that expression into 'Jimmy the One.'