Bombay duck

Uncertain. The first component may be a modification of Marathi बोंबील (bombīl), the local Marathi name for the fish. The second has been dubiously associated by folk etymology with dak (“Indian postal system”). This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term.

Bombay duck (plural Bombay ducks)

1. An edible lizardfish, Harpadon nehereus, found in the waters around Mumbai
2. (India, historical, slang) A member of the Bombay regiments of the East India Company's army.
   • 1872, Sir George Webbe Dasent, Three to One, page 42:

     For her sake he recounted his Indian experiences — told of his arrival in India; […] of the Bombay Ducks, that celebrated European regiment, of whom every man could drink a quart of rum a day, and fight a pitched battle if need were.

• (Harpadon nehereus): bamaloh, bombil, bumla, bommelow

• (member of Bombay regiment): 1873, John Camden Hotten, The Slang Dictionary