When you borrow something with the intent to not give it back. Less sever than stealing.
Do you like this book bag? I wagoner borrowed it from work.
The act of your best friend “borrowing” your sexual partner with approved consent, sort of like ‘sloppy seconds’. The reason it could be considered “borrowing” is because you DO give the partner back to your friend. (With ‘borrowing’, there would no financials involved.)
“Hey, bro, after you’re done in there, I’m borrowing.”