In a way that lasts forever or seemingly forever; eternally or persistently.
From everlasting (lasting forever) plus the adverbial suffix -ly. Everlasting comes from 'ever' plus 'lasting,' emerged around 1300, and has remained a favorite word for describing permanent or infinite things.
Medieval writers loved 'everlasting' for describing God, torture, and paradise, but adding -ly turned it into an adverb that English speakers use for everyday annoyances ('my little brother is everlastingly annoying'), showing how a sacred word became casual.
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