I'm thinking of when someone uses the phrase "flirting for tips" when describing women, and sometimes men, in the service money. People who will pay that little bit extra attention for that little bit more money.

Because it's always confused me when people use flirting in that context. Because to me flirting has to have sexual undertones to it and I can't see how either the customer or server can call it flirting when they both know that nothing will happen.