MOU is mostly used as "already" or "anymore," depending on the context. It's used to show frustration or resignation.
日本語を学ぶのは難しすぎるよ。もうやめる! - Learning Japanese is too difficult. Stop already!
私はそれをもう諦める. - I'm going to give it up.
もうこの男のことは諦めろ. - Jeez, just dump his ass already.
もうほっといてくれ!- Leave me the f*** alone already.
Other common examples:
もういいよ。- That's enough already.
もう知らないよ。- I don't know you anymore. (cutting someone out of your life)