出典:Wiktionary
From Middle English stilnesse, from 古期英語 stilnes (“stillness, quiet; absence of noise または disturbance, release, relaxation; silence, abstention from speech; absence of disturbance または molestation, tranquility, peace, security; that which appeases”), equivalent to still + -ness.
stillness (countable かつ uncountable, 複数形 stillnesses)