1Wasteful; what a waste - expressing regret at discarding or failing to make use of something. One of the most common everyday Japanese expressions, often used when encouraging saving or reuse.
まだ食べられるのに捨てるなんてもったいない。
It's such a waste to throw away food that's still good to eat.
電気をつけっぱなしにするのはもったいないよ。
Leaving the lights on is wasteful.
せっかく買った服を一度も着ないなんてもったいない。
It's a shame to never even wear clothes you went to the trouble of buying.
もったいないから、余ったご飯は冷凍しておこう。
It'd be a waste to throw it out, so let's freeze the leftover rice.
2Too good for one; more than one deserves - a humble expression used when receiving praise, a gift, or treatment that feels excessive for oneself.
こんなに立派な賞をいただけるなんて、もったいないです。
Receiving such a wonderful award is more than I deserve.
私にはもったいないくらい優しい奥さんです。
My wife is kinder to me than I deserve.
社長に直接お褒めいただくとは、もったいないお言葉です。
To be praised directly by the president is an honor I don't deserve.
3Impious or sacrilegious - an older meaning referring to treating something sacred with disrespect. Essentially obsolete in modern conversational Japanese; it survives mainly as a dictionary sense or in classical texts.