accomplice (n.)

"associate in crime," 1580s, an unetymological extension of earlier complice "an associate or confederate" (early 15c.), from Old French complice "a confederate, partner" (not in a criminal sense), from Late Latin complicem (nominative complex) "partner, confederate," from Latin complicare "to involve," literally "fold together," from com "with, together" (see com-) + plicare "to fold, weave" (from PIE root *plek- "to plait"). Altered perhaps on model of accomplish, etc., or by assimilation of the indefinite article in a complice.