What does Boxing Day have to do with boxing?
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.