Lieutenant combines the French ideas of place and holding, describing someone who acts for a superior during that person's absence. Its literal sense is a placeholder or deputy. Midshipman instead refers to where the person worked and berthed aboard ship. Different navies have used that title for a very junior officer or a very senior NCO, and some have used it for both.
Notable Point · 5:02-7:36
Lieutenants as deputies and the shipboard origins of midshipman
Lieutenant names the act of holding another person's place, while midshipman began as a description of work and berthing aboard ship.