also: りせいのたが
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.
In anger, the bounds of reason came off.
He's usually calm, but this time the limits of his common sense seem to have loosened.
Common modern spelling with タガ in katakana, clearly marking the metaphorical 'hoop' sense.
Kanji spelling using 箍 (hoop/barrel ring); less common but still seen, especially in literary contexts.
Hiragana spelling; readable but less common than the katakana form.
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.