In the book it says:
Jack the Lad is like a name for cheeky boys.
And new swing is the kinda music they were into.
So Marvin's ex-girlfriend came up with Jack The Lad Swing (JLS).
They were originally called:
UFO(Unique Famous Outrageous)
but Simon Cowell told them to change it in Bootcamp.
Glad to be of help! (:
Jack the lad is a really old saying meaning like a man who is a bit cheeky, has loads of girls that like him. Cool and like hangs around with his friends and has like a joke. And swing just makes it a bit more music like.