apple of discord

Something that causes unhappiness or trouble. That stupid prom dress has been their apple of discord—they're been fighting over who gets to wear it for days now! Why aren't you talking to Mary Beth? What is the apple of discord here?

Common Names:

Name	Pronounced	Usage
Abhishek	-	Indian, Hindi, Marathi, Gujarati, Punjabi, Bengali, Kannada, Telugu, Malayalam, Tamil
Paca	PAH-kah	Spanish
Eilwen	-	Welsh
Henryk	HEN-rik	Polish
Ru	-	Chinese
Behnam	-	Persian