Isabel Kloumann and a group of mathematicians at the University of Vermont published a paper in 2012 on positivity in the English language. They took just over 10,000 of the most frequent English words from a variety of sources (Twitter, Google Books, The New York Times, and music lyrics) and had people rate them on a 9 point scale from least happy to most happy, collecting 50 independent ratings per word. In the resulting dataset, available here, "laughter" comes in at number 1 in perceived happiness, and "terrorist" comes last.