Green’s Dictionary of Slang

too much, excessive.

1886		‘’Arry on Commercial Education’ in Punch 26 Sept. in P. Marks (2006) 124: Lop-sided free trade is all boko, and that’s why the Sossidges wins.
1889		‘’Arry in Switzerland’ in Punch 5 Dec. in P. Marks (2006) 98: But too much of yer Halpine Club bizness is boko, and not in my line.
1891		in Punch 21 Feb. 87: The Lights o’ London? Yah! That’s bin all boko.
1933		(con. WWI) F. Richards Old Soldiers Never Die (1964) 148: ‘Oh, Monsieur Nutty, Allemenge Boko Bombard.’ ‘Yes [...] and in one minute Monsieur Nutty Boko Bombard too.’.