something you say to someone you don't know at all, or you don't want to know at all.
a bull shit way to avoid talking anymore.
there's no thought put into what you actually want the person to have a good one of...
"Have a good one" being an alias for "Have a good wank", or more precisely "Go f*** yourself".
It seems that some people who are store clerks are embittered by the fact that they are store clerks and seek solace in this ambiguous flippancy.
VIP: Nice suit you are wearing eh?
Clerk: Thank you sir
VIP: Could you call me a cab?
Clerk: yes sir
VIP: good bye and have a nice day
Clerk: have a good one sir