also: りせいのたが
expression
the bounds of reason; the limits of common sense
Metaphorical expression likening reason to a barrel hoop that holds things together; often used when someone's self-control or rationality is about to break.
怒りで理性のタガが外れた。
In anger, the bounds of reason came off.
彼はいつも冷静だが、今回は理性のタガが緩んだようだ。
He's usually calm, but this time the limits of his common sense seem to have loosened.
A related expression meaning 'to lose self-control' or 'to go wild', using the same hoop metaphor without explicitly mentioning reason.
From 理性 (reason) + の + タガ (hoop/barrel ring). The metaphor is that reason acts like a hoop keeping a barrel together; when the hoop comes off, control is lost.