1
general::
Phrase(s): *light as a feather [and] *light as air
Cliché light in weight; [of cakes and pastries] delicate and airy. (*Also: as ~.) • Carrying Esther from the car to the house was no problem; she was as light as a feather. • What a delicious cake, Tom! And light as air, too.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Extremely lightweight. This simile can be used to refer either to physical weight, as in This load is light as a feather, or to texture, as in This cake is light as a feather. [Mid-1500s]
American Heritage Idioms