A happiness pump is someone who will do anything to increase other people's well-being even if it reduces their own profoundly.
The happiness pump is a person who has taken utilitarianism too far and will give themselves great pain so long as they believe it makes other people somewhere in the world much happier.
[1] Philosopher Joshua David Greene says it is almost impossible for a happiness pump to exist in real life because anyone who tried would give up very shortly.
[2] A happiness pump character, Doug Forcett, appears in one episode of the television show The Good Place.
[3] He's a man who accidentally received an insight into the rules of the afterlife while using psychedelic drugs, and he decides to have a torturously ascetic life devoid of anything that could cause suffering or even inconvenience of other living beings.