尾びれ背びれが付く

[cmertb]

Can't quite figure out what this expression means, and Google isn't being helpful.

The full quote is: 母ちゃん達の間で尾びれ背びれの付いた悪い噂がいっぱい流れてるんだ！！　今すぐにでも押し掛けてくる勢いだぞ！！

Could this be related to the bad rumor "floating about" so well that it even sprouted fins? Or is there some specific meaning to the expression?

[mikkih]

The full quote is: 母ちゃん達の間で尾びれ背びれの付いた悪い噂がいっぱい流れてるんだ！！　今すぐにでも押し掛けてくる勢いだぞ！！

(話に)尾ヒレがつく　(hanashi ni) ohire ga tsuku 　　is a set phrase.
It means (The story) gets blown up, or get exaggerated

To make it sound funnier, the manga-ka changed 尾ひれ(ohire) to 尾びれ(obire/a caudal fin) and added 背びれ(sebire/the dorsal fin).

[cmertb]

Ah! I should've known. I had to look up 尾ヒレがつく just the other day.

Thank you, mikkih!