Green’s Dictionary of Slang

a general negative intensifier, very, exceedingly; a euph. for bloody adv.

1900		Flynt & Walton Powers That Prey 183: I’m gettin’ bally tired o’ hearin’ you whine.
1933		Eve. Post (N.Z.) 17 June 12/6: ‘To bally well be or not to bally well be, that is the bally question’.
1938		N. Lindsay Age Of Consent 126: Have to get out of here [...] Man goes clean off his crumpet, arsing about a bally lonely hole like this all day with nothing to do.
1945		D. Bolster Roll On My Twelve 32: He thought while he was about it he’d get a bally great noggin so that he could carry up enough tea for the watch on deck in one.
1981		G. Johnston Fish Factory 34: Make a bally poor vegetarian, I would, Missus.
2004		P. Howard PS, I Scored the Bridesmaids 63: I’m going to bally well do it.