A Ninja-Lunch is when your co-workers start talking about where to go for lunch, and while you give some token input, like, "The pub" or "The donair place on the hill", you really have no intention of going to lunch with the boys. You've already secretly planned lunch with a redhead, and while the boys' backs are turned, you vanish like a ninja to meet your REAL lunch date.
Dave: Where should we go today?
George: The Old Admiral?
Yoon: How about Burgers?
Joe: Alright guys, time to go!!
Mike: Wait, where'd Brian go?
Yoon: He must have gone on a ninja lunch again.