Thin
Diluted, Faint
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What adjective describes something that's been diluted? Thin, diluted, or faint.
薄い is mostly for flat things, like 薄い紙 (thin paper) or 薄い壁 (thin walls). Stuff without depth, basically, which is why it also extends to expressions like 薄い味 (diluted flavor) and 薄い色 (faint color).
You take a sip of your coffee and find it's thin and diluted, so you say, "Ooh, Sue (うす)… This coffee isn't good." Sue's the one who made your coffee, but she's not much of a barista. "Ooh, Sue… I'm sorry, but this is way too diluted — I'm going to need you to brew a fresh pot."
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.
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