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1.
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"

OR

"Check out the layby."
by K.C.D.L. May 26, 2004
 
2.
A place in the road where it is legal to pull over and stop for a short period of time. Also can be used as a place where two consenting adults may participate in sexual acts in a public area.
Driver: Oh look a lay-by... i'm feeling frisky!
Passenger: Pull over then, I'll sort that out!
Driver: But there is already a car there?
Passenger: Don't worry i'll put on a good show!
by emma86 November 15, 2006
 
3.
When a chick is hot but couple of year younger than you, you put a “lay-by” on her so when she’s older, you can be in first!
wen ur mates lil sis is all that but a lil too young, u lay by her and be the first to get to her when shes old enough
by lil miss vixen April 21, 2005