Used in some parts of Australia like Canberra. A hot girl that's too young for you and hence you have to figuratively put her on layby. Usually used as a qualifier so you don't sound like a pedophile.
Guy 1:"Check her out."
Guy 2: "Dude, are you sick? She's gotta be like 13."
Guy 1: "Okay then she's a layby." or "I'll put her on layby"
"Check out the layby."