A slang expression meaning to be incarcerated in prison, with reference to the fact that one does not do much of any importance in prison except wait and count the time until the apointed date of release. Metaphorically, any unpleasant or boring situation which can only cured by waiting for it to pass.

Also Doing <unit of time>, eg Doing six months for shiplifting, as in the old joke:
A: I say, I haven't seen you in five years, what have you been doing?
B: Five years.