Kunukku (Akkadian), KiĊĦib (Sumerian) is the term for the cylinder seals used approximately until the overthrow of the Assyrian Empire.
The operation of the Kunukku/Kisib was that an item would be covered in clay and the cylinder seal would be rolled over it creating a continuous impression. This would be harder to counterfeit if broken than would a stamp seal.
The Kunukku could be impressed on clay over the stopper of an amphora of wine, for example; then allowed to dry before shipment. It could also be impressed over twine tying a door shut, thus indicating whether the premises had been entered. As a speculation, a Kunukku with the image of a god might be seen as invoking the god's protection.
The customary materials used were Lapis and Carnelian, worked by a Bur-gul or seal maker.
Contracts were entered on a clay tablet stating the agreement. Those who could attest to the agreement would also be noted and append a seal impression. This was then fire cured into a "brick", which was subsequently covered with clay and seal with a one-time Bur-gul seal Kunukku made of clay, which would be smashed after the use. If a dispute arose, the time cured clay covering could be broken, the witnesses brought forward and the issue resolved.
Other meanings
Kunukku is an ancient Christian Keralite type of jewelry. Usually it is an earring made of gold. It consists of a circular thin chain with a small ball hanging from it.
Imagine the shape of an upside-down water droplet with a ball hanging at the end of it.