admix (v.)

c. 1600, "act of mingling," with -ure + admix (1530s), a back-formed verb from admixt "mingled" (early 15c.), a past-participle adjective from Latin admixtus "mixed with," past participle of admiscere "to add to by mingling, mix with," from ad "to" (see ad-) + miscere "to mix" (from PIE root *meik- "to mix"). In Middle English admixt was mistaken as the past participle of a (then) non-existent *admix. The earlier noun was admixtion (late 14c., from Latin admixionem).

also *meig-, Proto-Indo-European root meaning "to mix." 

It forms all or part of: admix; admixture; immiscible; mash; meddle; medley; melange; melee; mestizo; metis; miscegenation; miscellaneous; miscible; mix; mixo-; mixture; mustang; pell-mell; promiscuous.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit misrah "mixed;" Greek misgein, mignynai "to mix, mix up, mingle; to join, bring together; join (battle); make acquainted with;" Old Church Slavonic mešo, mesiti "to mix," Russian meshat, Lithuanian maišau, maišyti "to mix, mingle," Welsh mysgu "to mix."