An onomatopoeic/mimetic word meaning that things are reversed, inverted, or back-to-front; can function as a noun or na-adjective.
棚の中の本があべこべに並んでいる。
The books on the shelf are arranged all mixed up/back-to-front.
説明があべこべで、何をすればいいか分からない。
The explanation was all mixed up, so I don't know what to do.
箱の中身があべこべになっていたので、元に戻した。
The contents of the box were all reversed, so I put them back.