melee n.

early 14c., "to mingle, blend, mix" (a sense now obsolete), from Old North French medler (Old French mesler, 12c., Modern French mêler) "to mix, mingle, to meddle," from Vulgar Latin *misculare (source of Provençal mesclar, Spanish mezclar, Italian mescolare, meschiare), from Latin miscere "to mix" (from PIE root *meik- "to mix").

From late 14c. as "busy oneself, be concerned with, engage in," and in the disparaging sense of "interfere or take part in inappropriately or impertinently, be officious, make a nuisance of oneself" (the notion is of meddling too much), which is the surviving sense of the word. From mid-14c. to c. 1700 it also was a euphemism for "have sexual intercourse." Related: Meddled; meddling.

c. 1300, "hand-to-hand combat, war, battle," a sense now obsolete, from Old French medlee, variant of meslee, from mesler "to mix, mingle, meddle" (see meddle). From mid-14c. as "cloth made of wools dyed and mingled before being spun," whether of one color or many, but especially pied cloth. The general meaning "a combination, a mixture" is from c. 1400; that of "musical composition or entertainment consisting of diverse parts from different sources" is from 1620s.

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."