So, I was at the bar last week,
Behind the bar they have these signs with jokes on them. One of them said "If your drinking to forget, please pay me first"(This isn't the question, I get this joke).
Somebody at the bar, noticing it, said to the bartender: "I don't have a drinking to forget, but if I did, what about it?".
The bartender, looked confused, and then the guy pointed to the sign. The bartender still didn't get it, and he laughed at her.
But I didn't get the joke either. This sentence doesn't make any sense to me. Why is it funny?