Definition of barn noun from the Oxford Advanced American Dictionary

barn

noun
/bɑrn/
 
 
1 a large farm building for storing grain or keeping animals ina hay barnThey live in a converted barn (= a barn that has been turned into a house).2 a large, plain, ugly buildingThey live in a big barn of a house.3 a building in which buses, trucks, etc. are kept when not being usedIDIOM

close, etc. the barn door after the horse has escaped

to try to prevent or avoid loss or damage when it is already too late to do so
close, etc. the barn door after the horse has escaped