Have a good one is a friendly farewell wishing somebody a good day. Thanks, have a good one.
The one is deliberately vague so the same phrase works morning or evening. It is common in shops and cafes and is American in origin.
1phrasehave a good oneA friendly farewell wishing somebody a good day.
“She handed back the change and said have a good one.”
have a good one is a farewell.
Practised in Small Talk & Everyday Phrases