陰口 [かげぐち] – gossip; talking about someone behind their back

• 陰 [イン・かげ] – shade; shadow; secret; yin
• 口 [コウ・くち] – mouth

If you have a secret, shady mouth, you might be talking about people behind their backs.

You’ve like already seen 陰 in the form 陰陽.  In Japanese, it is pronounced いんよう, but you’re probably more likely to recognize it as it’s read in Chinese: Yin/Yang.