熟
ジュク, う.れる
mellow, ripen, mature, acquire skill
JLPT1 G6 S15 F1415
As per
孰
# (an enclosure) +
火
fire → stir meat and other ingredients in a cooking vessel (compare
毒
and 融) → soften → *ripen*; *mature*.
[Full Kanji Etymology List]