酢
す · su · JLPT N3 · noun
vinegar
🍶 su = the first syllable of sushi, which means sour (vinegared) rice
Sushi takes its name from su-meshi, vinegared rice, so the su you already know from sushi is this word. The kanji 酢 has the 酉 (alcohol jar) radical on the left: vinegar is what fermented alcohol turns into when it goes sour.