Feather weight

(Racing)	Scrupulously exact weight, so that a feather would turn the scale, when a jockey is weighed or weighted.
	The lightest weight that can be put on the back of a horse in racing.
	In wrestling, boxing, etc., a term applied to the lightest of the classes into which contestants are divided; - in contradistinction to light weight, middle weight, and heavy weight.- Youatt.