酸
サン, す.い
acid, bitterness, sour, tart
JLPT1 G5 S14 F1218
As per 夋# as described in 俊 (stand erect) + 酉 alcohol → *sour* alcohol that, when imbibed, causes the body to stiffen (= become erect) in revulsion → *acid* (← sour or souring agent) → *severe*.
[Full Kanji Etymology List]