Hello, I am Yoshi from NILS Japanese language school. Today’s phrase is Mottainai もったいない wasteful.
Mottainai もったいない is often used as follows:

[Example]
Gohan o tabenai de suteru no wa mottainai.
ごはんを食べないで捨てるのはもったいない。
It’s wasteful to throw away uneaten rice.

Mottainai もったいない implies that the object wasted is something valuable that should be utiliezed to the fullest. It often carries a reproachful tone.
Mottainai describes an act or an action, never a person, whearas English “wasteful” could mean “wasting” and may modify a person, as in “He is a wasteful man.” To express the same idea in Japanese, however, one would have to say the following instead:

[Example]
Kare wa rohiteki na otoko da.
彼は浪費的（ろうひてき）な男だ。
He is a wasteful man.

March 9th News, from NILS Japanese language school.