Green’s Dictionary of Slang

(UK black) a term of surprise and shock, synon. with ‘What a lark!’ or ‘Wow!’ or ‘Isn’t that incredible!’.

2001

(con. 1981) A. Wheatle East of Acre Lane 76: ’Pon de front line is pure undercover beastman, to blowoh. An’ you can’t tell de coconuts from de pure chocolate, man. [Ibid.] 209: Blowoh. So dat’s why you did wan’ to check me.