Green’s Dictionary of Slang

a candidate for holy orders.

1836		Yorks. Gaz. 21 May 4/3: Si Quis knows nearly as much about logic as the mites in a cheese know of last Sunday’s eclipse.
1864, 1867, 1870		Hotten Sl. Dict.
[1864		Carlisle Jrnl 4 Mar. 9/5: If no objection had been raised when the ‘Si Quis’ was read, further opportunities were afforded at his ordination as a deacon].
1873		Sl. Dict.
1917		Buchan Obs. 23 Jan. 5/1: ‘Si Quis’ must not permit himself to become contempotuous of theories nor to take a too determined stand against dogmas.