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-).
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Harper, D. (n.d.). Etymology of toc. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/toc