also: つうといえばかあ
expression
quick to take a hint; very responsive; quick on the uptake
Describes a relationship or person where a brief cue (like 'tsu') immediately gets the right response ('ka'). Often used for close friends or smooth teamwork.
See also: ツーカー
あの二人はツーと言えばカーの仲だ。
Those two are so in sync that a single word gets an instant understanding.
彼はツーと言えばカーで、説明がほとんどいらない。
He's so quick on the uptake that he barely needs any explanation.
From the idea that saying 'tsu' immediately prompts 'ka' in response, mimicking a quick, intuitive exchange. The exact origin is uncertain, but it's a colloquial metaphor for smooth communication.