Calling someone a loaner douche is to imply that someone is a douche (or douchebag), but to an -extreme- degree. It means that the douche is SUCH a douche that they aren't even a FRESH douchebag. They are SUCH a horrible douche, that it seems as though they got the douche loaned to them from someone else who had already used it.... a nasty, disgusting -loaner- douche. It can also be used to agree with someone else being called a douche. This word is to be used with extreme caution, as only the most doucheyest of douches really deserve to be called a loaner douche. It first originated in Avon, IN amongst a group of friends.

Literally everyone, ever. Including the writer of this definition.

Loaner-douche is just another way to write loaner douche.
*See loaner douche for definition.