This word is used in the Bible, and I came across it in my studies the other day. To be at enmity with someone is to be at odds with them. The use of the word “hatred” in the Merriam-Webster definition above would seem to be a bit harsher than I would normally attribute to this word.
Another definition would be: the state or feeling of being actively opposed or hostile to someone or something.
That is more in line with what I intend when I use this word.