薄める
うすめる · usumeru · JLPT N2 · verb-ichidan
to dilute
💧🥤 薄める = 薄い (thin, weak) + める (to make): to make thinner
薄い means thin, weak or pale, and the ending める turns it into to make so, the same pattern as 強い → 強める. The kanji 薄 also contains the water radical 氵, which fits watering something down.
薄 — grass radical 艹 + 溥 (pǔ, phonetic)
薄 is a semantic-phonetic compound. The grass radical 艹 on top hints at the original meaning — thin, sparse vegetation or undergrowth. The lower part 溥 supplies the sound. From 'sparse plants' the meaning broadened to 'thin', 'slight', or 'meager' in general use.