relationship of intuitive understanding; close rapport
Describes a relationship where two people understand each other so well that words are barely needed; often used of close friends or long-time partners.
Those two have such a close rapport that they understand each other just by looking into each other's eyes.
Having worked together for many years, we now have an intuitive understanding of each other.
The exact origin is uncertain, but ツーカー is thought to be onomatopoeic, mimicking the quick, smooth back-and-forth of effortless communication. The phrase is colloquial and likely emerged in modern spoken Japanese.