Noun: A warmer. (Anything to warm somebody up, including pieces of cloth or alcoholic drinks.)
Lämmike appears in our example in the partitive singular, lämmikettä
The other word in the phrase, piristystä, means ”a cheering up,” or perhaps a better translation here is ”jazz up!”
The basic meaning is that this coffee will warm you up and perk you up. A better colloquial translation of the slogan ”lämmikettä, piristystä!” is very welcome.