"cold," early 14c.; see a- (1), here perhaps intensive + cold (adj.). Or it might be literally "a-cooled," from the past participle of the verb acool "to take cold" (late Old English); "to make cold" (15c.).

"highest point," 1560s, from Greek akmē "(highest) point, edge; peak of anything," hence "prime (of life, etc.), the best time" (from PIE *ak-ma-, suffixed form of root *ak- "be sharp, rise (out) to a point, pierce"). In English it was written in Greek letters until c. 1620. The U.S. grocery store chain was founded 1891 in Philadelphia, Pennsylvania.

1590s, "act of acknowledging," from acknowledge + -ment. "An early instance of -ment added to an orig. Eng. vb." [OED]. Meaning "token of due recognition" is recorded from 1610s.

late 15c., "admit or show one's knowledge," a blend of Middle English aknow "admit or show one's knowledge" and Middle English knowlechen "admit, acknowledge" (c. 1200; see knowledge). Middle English aknow is from Old English oncnawan "understand, come to recognize," from on (see on (prep.)) + cnawan "recognize;" see know). "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.

British oral code for letter -a- in wireless and telephone communication, 1898; hence ack-ack "anti-aircraft" (gun, fire, etc.). Compare toc (-t-), emma (-m-).

"sub-acidic, slightly sour" (of cream of tartar, oranges, etc.), 1766, also used figuratively for "sour-tempered;" from Latin acidulus "slightly sour," a diminutive of acidus (see acid (adj.)).