Definition of pennaceous in US English:

pennaceous

adjective

Ornithology
  • Relating to, of the nature of, or resembling a feather or feathers; (in later use) specifically designating (part of) a feather in which the barbs form a coherent vane (contrasted with downy or plumulaceous).

Origin

Early 19th century; earliest use found in James Francis Stephens (1792–1852), entomologist. From scientific Latin pennaceus + -ous.