(くさ)(えん)

くされえん
noun
inseparable bond; unbreakable (but unwanted) connection
informalemotion
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(くさ)(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. (くさ) is the continuative form of (くさ) (to rot), and (えん) means connection or fate. Not to be confused with purely negative relationships; it often describes friendships that have endured despite ups and downs. The expression implies a certain inevitability — this is a connection that fate (or habit) has made permanent.