in garrison
Webster's Dictionary (GCIDE)
Garrison \Gar"ri*son\, n. [OE. garnisoun, F. garnison garrison, in OF. & OE. also, provision, munitions, from garnir to garnish. See Garnish.] (Mil.) (a) A body of troops stationed in a fort or fortified town. (b) A fortified place, in which troops are quartered for its security. [1913 Webster]
In garrison, in the condition of a garrison; doing duty in a fort or as one of a garrison. [1913 Webster]