blackfriars!, excl. — Green’s Dictionary of Slang

blackfriars! excl.

[? the black uniform of various authorities, reminiscent of the black-garbed Dominicans]

(US Und.) a shout of warning, ‘someone’s coming, let’s run for it!’.

1857		‘Ducange Anglicus’ Vulgar Tongue 2: Blackfriars, v. Look out.
1859		Matsell Vocabulum.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.