The most common meaning of もったいない is "what a waste!" It often refers to wasting food or time etc. ex: I leave my food unfinished and therefore food will be thrown away. Then you can say the ward. 「（たべものが） もったいない！」 "What a waste (of food)!" I'm playing online games all day and do nothing besides that, you can say: 「じかんが もったいない！」 "What a waste of time! Also, you can use it as in "too good". ex: You bought an expensive gift for me, I would say: 「私にはもったいないです」, which means "that's too good for me". I think I don't deserve to receive the gift, that's why I say もったいない.

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