i-adjective
insufficient; not enough; lacking
Used when a quantity, quality, or condition falls short of what is needed or expected. Often appears with nouns marked by が or に.
See also: 足りる
時間が足りないので、急ぎましょう。
We don't have enough time, so let's hurry.
この説明だけでは情報が足りない。
This explanation alone doesn't provide enough information.
i-adjective
Colloquial and somewhat blunt; used to say someone is not very bright or lacks common sense. Often in phrases like 頭が足りない or 知恵が足りない.
あの人はちょっと頭が足りないんじゃないか。
Isn't that person a bit dim-witted?