Green’s Dictionary of Slang

ack emma n.

[orig. military use]

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

[UK]‘Ian Hay’ Carrying On 135: He [i.e. the signaller] salutes the rosy dawn as ‘Ak Emma,’ and eventide as ‘Pip Emma.’.
[Aus] 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.
[UK] in Manchon Le Slang.
[US] ‘F. Bonnamy’ A Rope of Sand (1947) 127: You haven’t watched bushes at two and three ack emma.
[UK]Wodehouse Mating Season 35: Wading in the Trafalgar Square fountain at five ack emma this morning.
[Aus]J. Wynnum I’m a Jack, All Right 21: Get yourselves back here on the dot of eight ack emma on the seventeenth.
[UK] Guardian Office 16 Dec. 8: Referring to [...] times as Ack Emma and Pip Emma (am and pm).