Green’s Dictionary of Slang

(UK prison) locked up.

1859		Hotten Dict. of Modern Sl. etc. 95: SLOWED, to be locked up — in prison.
1863		Story of a Lancashire Thief 8: I heard from a chum, who was fly to every move on the board, that a regular don was likely to turn up in Manchester; a swell prig who had hooked it from London to escape being slowed.
1873		Sl. Dict.
1909		C.B. Chrysler White Slavery 70: Can you imagine some poor ignorant ‘stiff’ going out and ‘prowling’ some house, takin’ a chance on gettin’ his ‘roof blowed off,’ or gettin’ ‘slowed’ and ‘slammed’ in the stir for a ‘ten spot’.