The pocket warmer, or also known as a poor-boy, is a device created to temporarily boost a sympathist's energy by producing a massive amount of heat.
They are described in the books as small, round, and metallic. They hold kerosene, naphtha, or sugar. When they are acivated the fuel burns and the sympathist has about 5 minutes of boosted heat until it dies out.
In The Wise Man's Fear Devi uses one to beat Kvothe in a duel. During the duel Kvothe reveals that Kilvin refers to them as "self-contained exothermic accelerators". He goes on to say they are messy, dangerous, and prone to breaking, and that they also need to be dismantled and cleaned between uses.