Thin, Weak, Dilute
い adjective
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 dilute? It's dilute. And something that's been diluted is now thin.
うすい
This drink is way too diluted. You sue (うす) the makers for their thin drink.
コピー機のインクの色が薄くなってきちゃった。新しいトナーを買わなくっちゃ。
My copier is printing with lighter ink. I have to buy new toner.