: Often used in the phrase "there's no such thing as a free lunch," meaning you can't get something for nothing. Can I Buy You Lunch? - GeoffMcDonald.com

Commonly used when someone wants to gain expert advice without paying a formal consulting fee.

Be careful not to confuse "buying lunch" with these similar-sounding idioms:

Offering a free meal in exchange for your time, knowledge, or insights.

It’s often seen as less formal and less intimidating than a night out, though it can still signal romantic intent. 4. Idiomatic Cousins