Green’s Dictionary of Slang

a (brief) term of imprisonment.

1890		Bulletin (Sydney) 16 Aug. 24/2: Next day the ex-Mayor was ordered a week’s ‘cells,’ with a ration of 12oz. bread and a half pint of water daily, for making a philosophical observation.
1933		Gibbons Truth About the Legion 105: It might, however, happen sometimes that even he would have to be arrested and then in that case he would do his cells like any other drunk.