kitchen door; back door; service door; tradesman's entrance
host's entrance to a tea-ceremony room
隣人が勝手口に野菜を置いた。
The neighbor left vegetables at the back door.
母は寝る前に勝手口の鍵を閉めた。
My mother locked the kitchen door before going to bed.
私は勝手口で牛乳配達の人が来るのを待った。
I waited at the back door for the milk delivery to arrive.