noun
borrowing without returning; keeping something borrowed
Describes the state of having borrowed something and not yet returned it, often with a nuance of neglect or irresponsibility. Commonly used with 〜にする or 〜だ.
彼に貸した本を借りっぱなしにされている。
He borrowed a book from me and hasn't returned it.
借りっぱなしはよくないよ。
It's not good to keep things you've borrowed without returning them.
借りる is the verb 'to borrow'; 借りっぱなし is the noun/adjectival noun describing the state of having borrowed and not returned.
返す means 'to return (something)'; 借りっぱなし implies the opposite—not returning.
Compound of 借り (continuative form of 借りる 'to borrow') + っぱなし (suffix indicating a state left as is, often with negative nuance).