腐(くさ)れ縁(えん)

くされえん
noun
inseparable bond; unbreakable (but unwanted) connection
informalemotionsociety
1. inseparable bond; relationship that persists despite one's wishes
A relationship or connection that continues even though the people involved may not always want it to. Often used with a tone of resignation or wry humor about a long-standing tie that simply cannot be severed.
あいつとは腐(くさ)れ縁(えん)だ。
That guy and I go way back — we're stuck with each other.
We've been tied together since elementary school and still see each other often.
No matter how many times we fight, we always end up back together. I suppose that's what you call an unbreakable bond.

Literally 'rotten connection' — a bond so deep it cannot be cut even if it has gone bad. Despite the negative literal meaning, it is often used with affection or humor about long-standing friendships. 腐(くさ)れ is from the verb 腐(くさ)れる (to rot, a variant of 腐(くさ)る), and 縁(えん) means connection or fate.