沸
フツ, わ.く, わ.かす
seethe, boil, ferment, uproar, breed
JLPT2 G7 S8 F1709
As per
弗
# (split) +
水
water → splitting away of bubbles (compare
泡
) on the surface of boiling water → *boil*.
[Full Kanji Etymology List]