be good. meaning

[American slang]
a departure response meaning "good-bye and behave yourself."
  Jane: Well, we're off. Be back in a week. Mary: Okay, have fun. Be good. Jane: Do I have to?
  Tom: Bye. Be good. Bill: See you next month.

• as good as:    1. The same as, no ...
• but good:    [American slang]se ...
• for good:    Adverb: for good&n ...

Examples

1. the walk will be good. get your gun. we'll both go.