"When one came to think of it, the Evil Forest was a fit home for such undesirable people."

The undesirables referred to are known as Efulefu in the Igbo language.  It translates literally to "woman" and was used as a derogatory name for worthless or weak men.  In this case, Okonkwo uses it to refer to the men that had abandoned the traditions of the tribe to embrace Christianity and join the European missionaries.