Wikipedia: Mottainai (もったいない?, [mottainai]) is a Japanese term conveying a sense of regret concerning waste. The expression “Mottainai!” can be uttered alone as an exclamation when something useful, such as food or time, is wasted, meaning roughly “what a waste!” or “Don’t waste.” In addition to its primary sense of “wasteful”, the word is also used to mean “impious; irreverent” or “more than one deserves”.
Mottainai is an old Buddhist word, which has ties “with the Shinto idea that objects have souls.” Mottainai has been referred to as a tradition, a cultural practice, and an idea which is still active in the Japanese “cultural DNA”, which has become an international concept.
Mottainai in Japanese refers to more than just physical waste (resources). It is even used to refer to thought patterns that give rise to wasteful action. Grammatically, it can be used in Japanese as an exclamation (“mottainai!”) or as an adjective phrase (“it feels mottainai”). There is no plural form. The collection of mottainai things could be called mottainai koto (もったいない事?).
As an exclamation (“mottainai!”) it means roughly “what a waste!” or “Don’t waste.” A simplistic
English equivalent is “waste not, want not”. A more elaborate meaning conveys a sense of value and worthiness and may be translated as “do not destroy (or lay waste to) that which is worthy.” However, mottainai is in fact an example of a word—like sukiyaki and sushi—that cannot really be translated into other languages and thus is used as it is.