1. Lacking physical or mental coordination.
2. Lacking planning, method, or organization.
In other words: Anyone, but usually a girl or funboy who has the inability to kick a football or throw a ball without looking like a complete retard.
After noticing a drunken lass has her mini skirt tucked into the back of her g-string and is presenting her ass cheeks to the rest of the world you turn to your mates and say "that lass is well unco"