Filthy
Dirty
noun, な adjective
Not pure things are filthy and dirty. Ewughh.
ふけつ
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.
「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."