Literally: my friend.
The Navajo word ak’is means friend, so shí, ní, or bí is attached to mean my, your, or his (her). This word is essentially used to describe siblings and maternal cousins, but in all of these cases it only applies to people of the same sex.
Modern usage has somewhat allowed the word to be applied to friendships between men and women. But, in the old days, you simply could not say that a woman was just your friend (if you were a man), or that a man was just your friend (if you were a woman). Especially when it was not uncommon for men to have multiple wives.