Green’s Dictionary of Slang

a.m.; in ext. use, the morning.

1909		People’s Pop. Mthly (IA) 1 Feb. n.p.: A. M. or P. M.—the ‘ack emma’ or ‘pip emma’ of the [British] army signallers.
1917		‘Ian Hay’ Carrying On 135: He [i.e. the signaller] salutes the rosy dawn as ‘Ak Emma,’ and eventide as ‘Pip Emma.’.
1918		R. Hugh Knyvett ‘Over There’ with the Australians 234: Lying snugly in a car wrapped in many blankets, and only disturbed by [...] the nurse rousing me at six ‘ack emma’ (A. M.) to have my face and hands washed.
1923		in Manchon Le Slang.
1944		‘F. Bonnamy’ A Rope of Sand (1947) 127: You haven’t watched bushes at two and three ack emma.
1949		Wodehouse Mating Season 35: Wading in the Trafalgar Square fountain at five ack emma this morning.
1967		J. Wynnum I’m a Jack, All Right 21: Get yourselves back here on the dot of eight ack emma on the seventeenth.
2002		Guardian Office 16 Dec. 8: Referring to [...] times as Ack Emma and Pip Emma (am and pm).