The quality or state of being billowy; the characteristic of rising and swelling like billows or waves.
From 'billowy' plus the abstract noun suffix '-ness,' creating a noun that names the quality itself. The chain is: Old Norse 'bylgja' → billow → billowy → billowiness.
Billowiness is a word almost exclusively used in poetry and literary description—linguists call words like this 'nonce words' or 'poetic words' because they exist mainly in the realm of imagination.
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