Secondhand
Used
noun, の adjective
The combination of kanji doesn't make perfect sense, but it's alright. Take something that's middle old and you have something that's used or secondhand. It's not 100% old yet... it's still in the middle, meaning you can give it to someone else to use, like your younger siblings.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
このベンツは中古です。
This Benz is secondhand.
中古でジーンズをかった。
I bought used jeans.
中古ショップには本やマンガがたくさんありますよ。
There are many books and manga in secondhand shops.