To Be Diluted
To Be Diminished, To Become Weak
intransitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji means dilute, and this is an intransitive verb, as you can guess from the まる ending. So this verb means to be diluted, to be diminished, or to become weak.
Since this word has okurigana (hiragana attached to the kanji) you know that it's probably going to be the kun'yomi reading.
The reading comes from 薄い, so if you know that you'll be able to read this too. If not, here's a mnemonic to help you:
You don't want your drink to be diluted, so you sue (うす) anyone who dares pour you a weak drink.
効果が薄まるので、お酒と一緒には飲まないで下さい。
Please don't take this with alcohol as its effectiveness will be diminished.
水で薄まったピンクの絵の具がパレットに残っていた。
The pink paint, diluted with water, was left on the palette.
たった一つの発言でせっかくの手紙のありがたみが薄まった。
With just one remark, the inherent value of the letter was diminished.