To be ‘as light as a feather’ means to be very light in weight.
Example of use: “Didn’t you pack anything? Your luggage is as light as a feather.”
No one is quite certain what the origin of the idiom “as light as a feather” is, but its meaning is quite literal. This idiom is very closely related to the term “featherweight,” which dates back to 1812. The term was originally used in horse racing, and was the lightest weight allowable; it evolved from the simpler term “feather,” which was used as early as 1760 to denote the lightest burden allowable in horse racing.