Dirty
Noun and な adjective
ふけつ
「わっ！君にビールをこぼしちゃったよね！本当にごめんなさい。」「大丈夫よ！このスカート、妹のなんだけど、妹はそもそも不潔だから多分気にしないと思うわ。」
"Ah! I just spilled beer on you, I’m really sorry." "It’s okay! This skirt is actually my sister’s and she’s pretty filthy anyway, so she probably wouldn’t care."
Not pure things are filthy and dirty. Ewughh.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.