From the book Bloom by Wil McCarthy.
In Bloom a Witch's Tit is a device thrown into a bloom of Mycora to slow or stop it's growth.
Mycora are nanomachines that use ambient energy (usually heat) and any and all materials around to self-replicate. Mycora are not under human control, so the only way to stop a grey goo scenario is to deprive the Mycora outbreaks (or blooms, for the flower-fractal shapes they make) of energy.
The Witch‘s Tit somehow - it is never explained - drastically reduces the amount of heat in an area, and is used in the fashion of a grenade. My theory is that the Tit uses highly sensitive and efficient thermocouples to convert heat into some form of energy that the Mycora cannot use, like radio waves or ultraviolet radiation. Like all anti-Mycora weapons, the Witch's Tit is never 100% successful. It would only work effectively where it could do more damage to the Mycora than the good it would do by feeding the Mycora with its substance.
The name comes, presumably, from the attributed coldness of the bosoms of magical practitioners.