Meanwhile

google

ref

late Middle English: from mean3 + while.

wiktionary

ref

From Middle English menewhile, equivalent to mean +‎ while. Adverb is by ellipsis from in the mean while, noun being conventionally written as one word after the adverb.

etymonline

ref

meanwhile (n.)

also mean while, late 14c., "mean time, the interval between one specified period and another," from mean (adj.2) "middle, intermediate" + while (n.). From late 14c. as an adverb, "during or in a certain period of time." Properly two words as a noun but commonly written as one, after the adverb.