Green’s Dictionary of Slang

a very short time, lit. ‘half a moment’.

1896		Windsor Mag. 4 117: Half a mo’, half a mo’! I’m king pin on this outing and you c’n give your advice when it’s asked.
1905		‘Doss Chiderdoss’ ‘Good For Trade’ Sporting Times 25 Mar. 1/4: Love’s old sweet songs attuned, in something less than half-a-mo’, / To the no less sweet refrain of ‘almond rock’.
1914		Bateman & Trevor [perf. Harry Champion] ‘Dr Shelley’ 🎵 I cured his wife in half a mo.
1924		C.J. Dennis ‘A Holy War’ in Chisholm (1951) 78: ‘’Old on,’ I sez. ‘Just let me think for one / Brief ’arf-a-mo. I’d love a crack or two / At this flash gun.’.
1932		(con. WWI) L. Mann Flesh in Armour 200: ‘Here, sir, half a mo. Whats that cove got that big [...] collar round his neck for?’.
c.1939		B. Bennett ‘Me and a Spade’ in Billy Bennett’s Fifth Budget 13: There were Esquimaux, half a mo’s, Ikey mo’s too.
1939		E.F. Benson Trouble for Lucia (1984) 46: ‘Now half a mo’ more,’ she said seizing her sketchbook.
1995		(con. 1860s) P. Ackroyd Dan Leno and the Limehouse Golem 152: I developed my own masculine slanguage. ‘’Arf a mo’ cocky.’.