"Adam shook his head. "No troops," he sighed. "I told my Chief about it, and he said we must wait till they chopped a white man. He advised me if ever I felt like it not to commit a--a barren felo de se, but to let the Sheshaheli do it. Then he could report, and then we could mop 'em up!"

I think "felo de se" = suicide. I'll search for it.

A felon of himself; a self-murderer.

To be guilty of this offence, the deceased must have had the will and intention of committing it, or else he committed no crime. As he is beyond the reach of human laws, he cannot be punished. English law used to inflict a punishment by a barbarous burial of his body, and by forfeiting to the king the property which he owned and which would belong to his relations.

You're right. According to Cecil Adams, it's a Latin legal phrase: see <A HREF="http://www.straightdope.com/classics/a1_139.html" target="_new">http://www.straightdope.com/classics/a1_139.html</A>. It seems more like French phrase. Our <i>felon</i> is probably of Germanic origin, via Anglo-Norman.
Yup, just looked <i>felo-de-se</i> up in the OED. It's described as Anglo-Latin.