りせいのたが· also りせいのタガ
the bounds of reason; the limits of (common) sense
Also written 理性の箍、理性のたが
Expression
たまには理性りせいのタガを外はずしてみませんか。
Why not just let fly once in a while?
普段ふだんはとてもおとなしいナイスガイだが、理性りせいのタガがはずれると人格じんかくが豹変ひょうへんする二重人格にじゅうじんかく。
He's got a dual personality - usually a quiet "nice guy" type, but when he flips, his character changes.