blue pigeon
/ˌbluː ˈpɪdʒ(ɪ)n/noun
1Any of various chiefly tropical pigeons having bluish plumage; (now) especially pigeons of the genus Alectroenas, found on islands in the western Indian Ocean.
2 Nautical slang. The lead attached to the sounding line; the plummet. Now chiefly historical.
Phrases
to fly the blue pigeon
Origin
Late 17th century (in an earlier sense). From blue + pigeon.