A lunch you have where your meal is sitting between your legs (in your crotch) when driving. This meal is very common to busy people who have no time to sit down and eat so they drive and eat at the same time.
Person 1: "I have to get to work but i havent eaten yet!"
Person 2: "Run by the McDonalds drive through then"
Person 1: "Damn this is the 4th crotch lunch