Figuratively, a stick-in-the-mud (usually written with hyphens and pronounced as if it is one long word) is a person whose behavior resembles, well, a stick (of wood) impaled in mud: someone who is resistant to being pulled into a productive activity. Such a person is a loner who does not like group activities or may be actively hostile to them. In vocabulary, a "grouch" is similar.
Example: "Carol was inviting co-workers to a party. However, she did not even bother trying to invite Paul to the party. Carol knew Paul was a stick-in-the-mud who hated parties; even if Paul could be convinced to go, he would ruin the party with his negative attitude. As a result, Carol did not invite him at all."