expression, godan verb (-su)
expose one's faults; show oneself up
Idiom meaning to inadvertently reveal one's hidden weaknesses, mistakes, or true (poor) character, often through careless words or actions.
彼は自慢話をしているうちに、ついボロを出してしまった。
While bragging, he inadvertently showed his true colors.
テストで簡単な問
I made a mistake on an easy question on the test and exposed my weakness.