expression, godan verb (-su)
show one's true colors; give oneself away; expose one's faults
Idiomatic expression meaning to inadvertently reveal one's hidden nature, bad intentions, or a secret, often after pretending otherwise.
彼はうそをついたが、すぐに尻尾を出した。
He lied, but soon gave himself away.
あの政治家は、スキャンダル
That politician finally showed his true colors in the scandal.