An old English saying that generally means someone who "Isn't quite there" or is slightly loopy or crazy.
Another sayings similar to this are: "One sandwich away from being a picnic"
-boy pretending to play Halo in the middle of the lunch area and is rolling around pretending to shoot people-
My friend: "I wanna punch him!"
Me: "Nah man don't, he's not the full quid"
My friend: "Why is he talking to that pole?"
Me: "Because, he's not the full quid"