1550s, a blend of Middle English aknow (from Old English oncnawan "understand," from on + cnawan "recognize;" see know) and Middle English knowlechen "admit, acknowledge" (c.1200; see knowledge). In the merger, a parasitic -c- slipped in, so that while the kn- became a simple "n" sound (as in know), the -c- stepped up to preserve, in this word, the ancient "kn-" sound. Related: Acknowledged; acknowledging.