expression, godan verb (-ku)
become exaggerated; get embellished
Idiomatic expression literally meaning 'tail fins grow'. Used when a story, rumor, or report gets exaggerated or embellished as it spreads.
彼の話は尾ひれがついて、どんどん大げさになった。
His story got more and more exaggerated as it spread.
The rumor became embellished and ended up far from the truth.