Vacant, Futile, Vain
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of void? It's empty or vacant. Just think of what has to happen to make a void in the first place.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
If everything in life seems empty and futile, even cows will be affected. Listen to them say, "Moo, nah (むな), never mind. What's the point?" Nothing makes life seem empty like a cow saying "Moo, nah."
It's futile to just cry every day.
I feel empty today, for some reason.
Don't you feel empty doing the same thing every day over and over again?