Updated: SEPTEMBER 4, 2017
A metamour is a word used within the polyamory community to describe your partners' other partners. The prefix "meta" comes from the Greek meta which means "after" or "beyond." "Amour" is the French for "love" or "lover" from the latin amor.
In this case, you could translate metamour as "lover beyond my lover".
More About Metamour
In typical polyamorous relationships, your metamours are people who you do not have a loving relationship with. You may be close to them as part of your polycule, but you are not in a romantic or sexual relationship with them.
If you start dating your metamour, this person becomes one of your partners or lovers, and they cease to be only a metamour. This word is only used to describe the people your partners dating, and you may have metamours in common with other members of your polycule.