Green’s Dictionary of Slang

synagogue n.

[by late 19C, Covent Garden market was very much a Jewish enterprise]

a shed – its use is not specified – standing at that time in the northeast corner of Covent Garden, London.

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[UK]J. Ware Passing Eng. of the Victorian Era 238/1: Synagogue (Covent Garden, 1890 on). Shed in the north-east corner of ‘the Garden’. So called from this place (erected 1890) being ‘wholly ran’ by Jews.