1650s, "composed of interconnected parts, formed by a combination of simple things or elements," from French complexe "complicated, complex, intricate" (17c.), from Latin complexus "surrounding, encompassing," past participle of complecti "to encircle, embrace," in transferred use, "to hold fast, master, comprehend," from com "with, together" (see com-) + plectere "to weave, braid, twine, entwine," from PIE *plek-to-, suffixed form of root *plek- "to plait."
The meaning "involved, intricate, complicated, not easily analyzed" is first recorded 1715. Complex sentence, for one containing one or more subordinate clauses in addition to the principal clause, is attested from 1776.
1650s, "a whole comprised of interconnected parts," from complex (adj.). Latin completus as a noun meant "a surrounding, embracing, connection, relation." Psychological sense of "connected group of repressed ideas" was established by C.G. Jung, 1907.
It forms all or part of: accomplice; application; apply; complex; complexion; complicate; complication; complicity; deploy; display; duplex; duplicate; duplicity; employ; explicate; explicit; exploit; flax; implex; implicate; implication; implicit; imply; multiply; perplex; perplexity; plait; plash (v.2) "to interlace;" pleat; -plex; plexus; pliable; pliant; plie; plight (n.1) "condition or state;" ply (v.1) "work with, use;" ply (v.2) "to bend; ply (n.) "a layer, fold;" replica; replicate; replication; reply; simplex; splay; triplicate.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prasna- "turban;" Greek plekein "to plait, braid, wind, twine," plektos "twisted;" Latin plicare "to lay, fold, twist," plectere (past participle plexus) "to plait, braid, intertwine;" Old Church Slavonic plesti "to braid, plait, twist," Russian plesti; Gothic flahta "braid;" Old Norse fletta, Old High German flehtan "to plait;" Old English fleax "cloth made with flax, linen."
1620s, "to intertwine," from Latin complicatus "folded together; confused, intricate," past participle of complicare "to involve," literally "to fold together," from com "with, together" (see com-) + plicare "to fold, weave" (from PIE root *plek- "to plait"). Meaning "to make more complex or intricate" is recorded from 1832, from earlier sense "to combine in a complex way" (17c.). Related: Complicated; complicating.
mid-14c., complexioun, "temperament, natural disposition of body or mind," from Old French complexion, complession "combination of humors," hence "temperament, character, make-up," from Latin complexionem (nominative complexio) "combination" (in Late Latin, "physical constitution"), from complexus "surrounding, encompassing," past participle of complecti "to encircle, embrace," in transferred use, "to hold fast, master, comprehend," from com "with, together" (see com-) + plectere "to weave, braid, twine, entwine," from PIE *plek-to-, suffixed form of root *plek- "to plait."
The Middle English sense is from the old medicine notion of bodily constitution or general nature resulting from blending of the four primary qualities (hot, cold, dry, moist) or humors (blood, phlegm, choler, black choler). The specific meaning "color or hue of the skin of the face" developed by mid-15c. In medieval physiology, the color of the face was believed to indicate temperament or health. The word rarely is used in the sense of "state of being complex."