Why is the ninth month called September?
1640s, "tangled," from past participle adjective from complicate. Figurative meaning "not easy to solve, intricate, confused, difficult to unravel" is from 1650s.
1620s, "to intertwine" (as a past participle adjective, early 15c.), from Latin complicatus "folded together; confused, intricate," past participle of complicare (see complication). Meaning "to make more complex" is recorded from 1832, from earlier sense "to combine in a complex way" (17c.). Related: Complicated; complicating.