fem. proper name, from German Emma, from Erma, contraction of Ermentrude or some similar name. With lower-case -e-, as British telephone and radio enunciation of -m- (to avoid confusion with -n-), attested by 1891, common in British soldier's jargon in World War I, for example ack emma "a.m.," pip emma "p.m." Compare ack (-a-), toc (-t-).
<a href="https://www.etymonline.com/word/ack">Etymology of ack by etymonline</a>
Harper, D. (n.d.). Etymology of ack. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ack