'Complex' is Latin for 'woven together' — from 'plectere' (to braid). Complexity is a tangle.
Consisting of many different and connected parts; not easy to analyse or understand. As a noun, a group of similar buildings or facilities, or a related group of repressed feelings.
From Latin "complexus" meaning "encompassing, embracing," past participle of "complectī" (to entwine, to embrace, to comprise), composed of "com-" (together, with) and "plectere" (to weave, to braid). Latin "plectere" derives from Proto-Indo-European *pleḱ- (to plait, to weave), a root of remarkable breadth. PIE *pleḱ- produced Greek "plekein" (to plait, to braid), giving "plexus" and "perplexus" (thoroughly entangled); Old English "flax" (the plant whose fibers are woven — via Germanic *flahsą from *pleḱ-); Latin "plicāre" (to fold — a variant form giving "complicate," "replicate," "explicit"); German "flechten" (to braid, to weave); Old Church Slavonic "plesti" (to weave); and Sanskrit "praśna" (a question — originally a knot, something woven together). The word entered English in the 17th century, initially as an adjective in geometry and music
The psychological 'complex' — as in 'inferiority complex' or 'Oedipus complex' — was introduced by Carl Jung around 1907. He chose the word deliberately for its etymological sense of 'things woven together': a complex is a knot of associated feelings, memories, and impulses that are tangled so tightly they operate as a single unit in the unconscious.