He broke into the house because his girlfriend had locked him out.

locked 是动词 lock 的过去分词形式，lock 的意思是"锁上、锁住"。 out 是副词，表示"在外面"。 locked out 是一个短语动词，意思是"被锁在外面"，指被故意或意外地关在门外无法进入。