Why is the novel called The Absolutely True Diary of a Part-time Indian?

The story is fashioned after Sherman Alexie's own childhood. In many ways it is a diary and the stories/characters are based on his life events. Junior, the protagonist, straddles two worlds. He is a product of the reservation system yet he attends white school. Junior is literally a "part time Indian" because he learns to adapt to the white world while being an Indian when he returns to his reservation.