Green’s Dictionary of Slang

jack n.16

[? one is a ‘jack in a box’, i.e. box n.1 (2b) or on one’s jack (jones) under jack (jones) adj.]

(UK tramp) a day (in prison).

[UK]Leamington Spa Courier 20 Sept. 7/1: He had just come out of Warwick ‘stir’ (gaol) where he had been doing fourteen ‘jacks’ (days) for ‘gagging’.