Lately I've noticed a new expression that is used rather frequently. It's easy to guess that "Have a good one" is a form of saying "Have a good day", "Have a good morning", etc.

But it was interesting how this new expression appeared and what  those people for whom English is the native language think about it. So I've read some branches at linguistic forums and found some interesting posts there.

"Have we become this lazy? I mean, really, is it that difficult to keep track of whether it is morning, afternoon or evening?

If you have not yet eaten lunch then chances are that it is still morning.
If you have not yet eaten dinner than chances are that it is still the afternoon.
If you have not yet gone to bed then chances are that it is still the evening.
If you are still up and about after midnight then chances are that you are slurring your words anyway."

"As far as what it should mean from the cashier, I would think the most logical choice would be that she wants you to have a good day. But maybe when she says "have a good one" she only wants you to have ONE good day, the implication being that she wants all the rest of your days to be bad. Next time someone says such `a rude thing to you you should let 'em have it."