grayback

[grey-bak] /ˈgreɪˌbæk/
noun
1.
any of various marine and aquatic animals that are dark gray above and light-colored or white below, as the gray whale, the alewife, certain whitefish, and certain sandpipers.
2.
Informal. a Confederate soldier.
Origin
1805-15; gray1 + back1
British Dictionary definitions for grayback

greyback

/ˈɡreɪˌbæk/
noun
1.
any of various animals having a grey back, such as the grey whale and the hooded crow