How_do_you_do

Use it only when being introduced to someone for the first time.

The standard and most correct response is to repeat the phrase back: "How do you do?" [English Oxford Living Dictionaries]. how_do_you_do

It evolved from the Middle English "How do ye?" (meaning "How do you fare?" or "How is your health?"). By the 16th century, it solidified into the greeting used today. Use it only when being introduced to someone