fare dodging; riding without paying
A slang expression from the Edo period, based on a pun: 薩摩の守 (Satsuma no Kami, a title) sounds like さつまのかみ (Satsuma no kami), which can be interpreted as 'not paying' (from さつま, a dialect word for 'free', and かみ, 'biting' or 'cheating'). Used humorously for traveling without paying a fare.
In the Edo period, the word 薩摩の守 was used as slang for fare dodging.