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-miente (archaic) Adverb-forming suffix akin to "-ly."
Early 13th cent. Old Spanish -mientre, under contamination from -iter, and -miente. All but entirely replaced by -mente, which was originally a dialect variant. From Vulgar Latin *-mente.

"In modern Spanish, -mente is no longer perceived as a noun (its congener miente is now confined to a few idiomatic expressions), but it can still be detached from its host adjective in order to avoid awkward repetition.... ~ Harris & Vincent, The Romance Languages (2003)