summer fur; summer coat; summer plumage
Refers to the thinner, lighter coat or plumage that animals grow in warmer months. Often contrasted with 冬毛 (ふゆげ, winter coat).
Cats shed their winter coat and grow a summer coat in spring.
This breed has short summer fur.
Compound of 夏 (なつ, summer) and 毛 (け, fur/hair), with rendaku voicing け → げ.