undersky

From under- +‎ sky.

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undersky (plural underskies)

1. The lower region of the sky.
   • 1842, Alfred Tennyson, “The Dying Swan”, in Poems. […], volume I, London: Edward Moxon, […], →OCLC, page 54:

     [A]t first to the ear / The warble was low, and full and clear; / And floating about the under-sky, […]