broad and shallow, n. — Green’s Dictionary of Slang

broad and shallow n.

[Broad Church members of the Church of England, ‘who take its formularies and doctrines in a broad or liberal sense and hold that the church should be comprehensive and tolerant’ (OED)]

the ‘Broad Church’.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 105: ‘Broad and Shallow’ an epithet applied to the so-called ‘Broad Church,’ in contradistinction to the ‘High’ and ‘Low’ Churches.
1864, 1867, 1870		Hotten Sl. Dict.
1866		Letters by an Odd Boy 163: Why, in my theological opinions, should I be ‘high and dry,’ or ‘low and slow,’ or ‘broad and shallow?’.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.