an officer ranking next above second lieutenant and next below a captain.
Origin
1775-85
Examples from the web for first lieutenant
He was first lieutenant at the time, but his captain was detached on other duty.
He had been promoted to first lieutenant two days before, but in his typical understated manner kept the news to himself.
His presumably loyal first lieutenant is as vicious and sneaky a cad as a haughty can generously make him be.
British Dictionary definitions for first lieutenant
first lieutenant
noun
1.
the officer responsible for the upkeep and maintenance of a warship, esp the executive officer of a smaller ship in the Royal Navy
2.
an officer holding commissioned rank in the US Army, Air Force, Marine Corps, or in certain other forces, senior to a second lieutenant and junior to a captain