acknowledge (v.)

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.

Definitions of acknowledge

acknowledge (v.)
declare to be true or admit the existence or reality or truth of;
She acknowledged that she might have forgotten
Synonyms: admit
acknowledge (v.)
report the receipt of;
The program committee acknowledged the submission of the authors of the paper
Synonyms: receipt
acknowledge (v.)
express recognition of the presence or existence of, or acquaintance with;
She acknowledged his complement with a smile
He never acknowledges his colleagues when they run into him in the hallway
it is important to acknowledge the work of others in one's own writing
Synonyms: notice
acknowledge (v.)
express obligation, thanks, or gratitude for;
We must acknowledge the kindness she showed towards us
Synonyms: recognize / recognise
acknowledge (v.)
accept as legally binding and valid;
acknowledge the deed
acknowledge (v.)
accept (someone) to be what is claimed or accept his power and authority;
The Crown Prince was acknowledged as the true heir to the throne
Synonyms: recognize / recognise / know