Green’s Dictionary of Slang

(UK Und.) a person who is categorized as weak-minded; usu. in pl. the barmies.

1929		C.G. Gordon Crooks of the Und. 45: He must have been one of the ‘barmies’ [...] A weird person altogether.
1934		P. Allingham Cheapjack 40: The whole bunch of ’em was a lot of barmies from some loony bin.
1937		‘Red Collar Man’ ‘Chokey’ 92: What with barmies [...] and the not infrequent holy rows, chokey could be very lively.
1937		E. Raymond Marsh 373: You’re a barmy, ain’t yer? I like to look at barmies.