Seal
noun
The kanji is seal in, but the vocab noun version is seal. This is simply a seal (not the water kind, the kind you use to seal something with).
ふう
The reading is the same as one of the readings you learned: ふう. Don't be a fool, make sure you seal your envelopes.
自分が六歳の子どもだってことは分かってたけど、父さんが勝手に封を開けて友達からの手紙を読んだことにはすごく腹が立ったわ。
I know I was just a 6-year-old kid, but I got so mad when my dad opened the envelope and read the letter from my friend without asking me first.