수어지교가 · 수어지교를
an inseparable relationship, as close as fish and water
Example
두 사람은 어릴 때부터 수어지교 같은 사이예요.
The two have been as inseparable as fish and water since childhood.
Word parts
수 water+어 fish+지 of+교 friendship
Literally 'the friendship of water and fish' — Liu Bei's words about Zhuge Liang.