late 15c., "admit or show one's knowledge," a blend of Middle English aknow "admit or show one's knowledge" (from Old English oncnawan "understand, come to recognize," from on (see on (prep.)) + cnawan "recognize;" see know) and Middle English knowlechen "admit, acknowledge" (c. 1200; see knowledge). "By 16th c. the earlier vbs. knowledge and a(c)know ... were obs., and acknowledge took their place" [OED].
In the merger, an unetymological -c- slipped in; perhaps the explanation is that when English kn- became a simple "n" sound, the -c- stepped up to preserve, in this word, the ancient "kn-" sound. Related: Acknowledged; acknowledging.