What is the meaning of Kettling?

A crowd control technique, used by police, where a hostile group of protesters or rioters are surrounded and not allowed to disperse, sometimes with the use of temporary fencing to corral and isolate portions of the group.

The situation where water gets trapped in the heat exchanger of a boiler (e.g. due to limescale build-up), resulting in knocking sounds as it turns into expanding steam.