When you see a chick who is questionable to be of age. Usually a chick who is at high school level but can almost pass for a older woman. Its usually that chick you really have to sit and look at before you even have the thought of sex come to mind. The males version of "spider senses".
Guy 1: Man that chick is hot, I'd love to sit that in my lap and grind her out like good coffee
Guy 2: Eww dude, wtf. You live right next to a high school. She isn't suspect to you?
Guy 1: No...
Guy 2: Seriously, thats jail bait. You touch her in your imagination and you're fucked