What is "aye aye, sir"

phr. (context idiomatic nautical English) The correct and seamanlike reply, onboard a Royal Navy (or U.S. Navy) ship, on receipt of an order from someone of senior rank or authority. It means "I understand the command and hasten to comply with the order."

"Aye aye, sir" is a phrase commonly heard present day in naval language. It is derived from a duplicate of the word "aye" which came into the English language in the late 16th century and early 17th century, meaning "Yes; even so.".