Etymology derives from Ancient Greek for "stupid"; can also be used in reference to clumsiness and oafishness.
Strong correllation between a Colley and the tendency to wear clothes several sizes too big, perhaps to prevent further restriction of clumsy limbs.
Look over there, that bloke's just walked into a wall and knocked it over.
And have you seen the size of the guy's t-shirt?
It must be a Colley