The reading is the kun'yomi vocab readings for these two kanji, probably because this word is just two words put together into one (bad + mouth). Use your knowledge of the vocab words 悪い and 口 to learn the reading of this word.
Okay, I lied. You can also do わるぐち if you want. Sorry to say such bad things before.
Viet started shouting verbal abuse at Koichi with a malicious grin.
I've never bad mouthed my husband in front of my kids.
I heard that all of my friends were saying bad things about me.