In old children’s English, honest Injun was a way to swear you were telling the truth — like “I promise,” said to make someone believe you. You will meet it in old stories, not in kind talk today. Though actually, Injun itself is an old spoken form of Indian, used for Native American people, and that use is rude now. Children copied the phrase without thinking about that hurt; adults should not copy it. If you need to promise the truth, say I promise or honestly instead.

honest Injun：舊時小孩發誓「我說的是真的」。Injun 本身是 Indian 的俗稱，現已冒犯。