Definition of ツーといえばカーだ
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(exp) to be quick to take a hint; to be very responsive; to be quick on the uptake
彼とはツーと言えばカーの仲だよ。以心伝心の仲とでも言っておこう。
He and I have a near-telepathic understanding of each other. No sooner does one of us say something than the other is already responding.