Green’s Dictionary of Slang

1. a brothel.

1654		E. Gayton Pleasant Notes III ii 76: Those Countries were better furnished with She-Nurseries and Seminaries of no Religion.
1779		Nocturnal Revels I 15: Some of the first-rate Thais’s who afterwards figured in gay life, were trained in her seminary.
1904		Sun. Times (Perth) 20 Mar. 1/1: A casual friend told him he could score an ad. at the Stirling-atreet ‘Ladies’ School.’ [...] on seeking the ‘seminary’ he received a shock by being languished by Lu-lu.

2. (UK Und.) a criminal gang.

1850

Ulster Gaz. 23 Mar. 2/5: Tho only witness was James Collins, evidently one of the notorious ‘Hagan seminary,’ who bad been told by the parties accused that they had stolen the tobacco.

3. the vagina.

1890–1904

Farmer & Henley Sl. and Its Analogues.