promiscuous adj.

word-forming element meaning "forward, forth, toward the front" (as in proclaim, proceed); "beforehand, in advance" (prohibit, provide); "taking care of" (procure); "in place of, on behalf of" (proconsul, pronoun); from Latin pro (adv., prep.) "on behalf of, in place of, before, for, in exchange for, just as," which also was used as a first element in compounds and had a collateral form por-.

Also in some cases from cognate Greek pro "before, in front of, sooner," which also was used in Greek as a prefix (as in problem). Both the Latin and Greek words are from PIE *pro- (source also of Sanskrit pra- "before, forward, forth;" Gothic faura "before," Old English fore "before, for, on account of," fram "forward, from;" Old Irish roar "enough"), extended form of root *per- (1) "forward," hence "in front of, before, toward, near," etc.

The common modern sense of "in favor of, favoring" (pro-independence, pro-fluoridation, pro-Soviet, etc.) was not in classical Latin and is attested in English from early 19c.

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." 

1834, "indiscriminate mixture, confusion," from French promiscuité (1752), from Latin promiscuus "mixed, not separated" (see promiscuous) + French -ité (see -ity). By 1844 in the sense of "promiscuous sexual union" (originally as among races of people). An earlier word was promiscuousness (by 1773 general; 1808 sexual).