1650s, "composed of parts," 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" (see com-) + plectere "to weave, braid, twine, entwine," from PIE *plek-to-, from root *plek- "to plait" (see ply, v.1). The meaning "not easily analyzed" is first recorded 1715. Complex sentence is attested from 1881.

1650s, "a whole comprised of parts," from complex, adj.. Psychological sense of "connected group of repressed ideas" was established by C.G. Jung, 1907.