Meantime

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Middle English (as a noun): from mean3 + time.

wiktionary

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From Middle English menetime, equivalent to mean +‎ time. Adverb is by ellipsis from in the meantime.

etymonline

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meantime (n.)

also mean time, mid-14c., mene-time, "interim, interval between one specified time and another" (now only in in the mean time), from mean (adj.2) "middle, intermediate" + time (n.). Late 14c. as an adverb, "during the interval (between one specified time and another)." As a noun, properly written as two words but commonly as one, after the adverb. In the mean space "meanwhile" was in use 16c.-18c.