Green’s Dictionary of Slang

p.m.; in ext. use, the evening.

1916		‘New Church’ Times 22 May (2006) /2: I shall dine / At 8.15 pip emma.
1917		‘Ian Hay’ Carrying On 135: He [i.e. the signaller] salutes the rosy dawn as ‘Ak Emma,’ and eventide as ‘Pip Emma.’.
1922		Weekly Journal Miner (Prescott, AZ) 5 July 3/1: At one pip emma came the picturesque parade of polite punchers.