casual: 잘 가 · formal: 안녕히 가십시오
goodbye (to someone who is leaving)
Example
오늘 고마웠어요. 안녕히 가세요.
Thanks for today. Goodbye!
Word parts
안 peace+녕 peace
안녕히 (peacefully) + 가세요 (please go) — 'go in peace'.
Good to know
Say 가세요 to the person who is leaving. If you leave and they stay, say 안녕히 계세요.