What is the prison meaning?
Because of differing semantic coverage, a lot of words between German and English are not 1:1 mappings. A reverse example is that in English we often speak of generic entrances, while in German you would normally distinguish between an entrance you drive through ("Einfahrt") versus walk ("Eingang").
It's wider than that and covers also the sleeping place of people and animals. Lager is related to lair in English.