Maji (まじ): Really? You serious?

Makasete Kudasai (任せて下さい): Leave it to me. Entrust that to me.

Makeru (負ける): To be defeated. You will more often hear this as zettai makenai, or a variation of, which means “I wouldn’t be defeated!”

Mamoru (守る): To protect. Shouting minna o mamoru (to protect everybody) will more often than not, suddenly fill an Anime protagonist with incredible power.

Maniau (間に合う): To be in time. The negative form is maniawanai.

Masaka (まさか): Impossible! No way!
Mattaku (まったく): This is best understood as a mild expletive to express annoyance. Frequently pronounced without the first sound too.

Mazui (まずい): Adjective for something that’s highly troublesome or bad tasting.
Me No Mae Ni (目の前に): Literally, before one’s eyes.

Meccha (めっちゃ): Kansai adverb meaning “very.”

Meiwaku (迷惑): While the kanji suggests bewilderment, the word actually means annoyance, irritation, frustration, etc.

Mendousai (面倒さい): Troublesome. Like other Japanese words ending with “…ai,” it is often pronounced as mendouse.

Minna (みんな): Everybody.

… mitai (… みたい): A suffix meaning, “alike.” For example, inu mitai. (Like a dog)

Mochiron (もちろん): Of course.

Moeru (燃える): To ignite.

Mondai (問題): Problem.

Moshi Wake Gozaimasen (もし分けございませ): An elaborate Japanese phrase for “sorry.”

Moshikashite (もしかして): Could it possibly be …

Mou Genkai Da (もう限界だ): At my/his/its limits.

Muri (無理): Undoable, impossible, unachievable. Note that muri could also imply excess. As in, muri o shinai. (Don’t overdo it)

Nakama (仲間): Companion. Ally.

Naruhodo (なるほど): I see. The best phrase to utter when you have spectacularly deduced the criminal in a crime mystery.

… ni natta (… になった): Ni natta is the informal form of ni narimasu. It means “has become” or “has changed into.” For example, ookii ni natta. (It turned big)

Nigeru (逃げる): To escape.

Ningen (人間): Human.

Nioi (匂い): Scent.

… no koto ga suki desu (… のことが好きです): This Japanese phrase always follows the name of a person or entity, and is a declaration of love. Few high school rom-coms are without several tearful mouthings of this.

… no sei (… の せい ): Fault. Doraemon no sei. Doraemon’s fault.

… no tame ni (… のために): For the sake of. In Shonen Anime, this is almost always hollered by protagonists at the lowest point of a fight. For example, minna no tame ni! (For the sake of everyone!) Ai no tame ni! (For the sake of love!)

Nodo Ga Kara Kara (のどがカラカラ): I’m thirsty.

Nombiri Suru (のんびりする): To take it easy.

Oiishi (美味しい): Delicious!

Okama (おかま): Homosexual or cross-dresser.

Omae (お前): A very uncouth way of saying “you.” Strictly speaking, this pronoun should only be used on someone with a lower social or family status, and in a harsh context. In Anime, however, many male characters use it on everyone, friends and foes alike.

Omae Kankei Nai (お前 かんけいない): None of your business.

Omoshiroi (面白い): Interesting. The opposite is tsumaranai.