"Have a good one!"

As my co-worker left the bathroom she called "Have a good one." It struck me - what a funny saying...

What 'one' am I having a good one of? A good experience on the toilet, a good hour, a good afternoon, a good day (which she had already told me earlier this morning), a good night? It could mean so much. That statement is the Universal - have a good (fill in the blank).