crybaby
A person who overreacts and complains a lot about problems.
A crybaby is someone who complains, whines, or gets upset over small problems that most people would handle without much fuss. If your friend refuses to play basketball because they didn't get picked first, or storms off crying because they lost one round of a video game, they're acting like a crybaby.
The word combines cry and baby because it suggests someone is behaving like a young child who hasn't yet learned to handle disappointment. Real babies cry because it's their only way to communicate when they're hungry, tired, or uncomfortable. But when an older kid or adult acts this way over minor setbacks, it can seem immature.
You might hear someone called a sore loser or a crybaby after they make excuses, blame others, or throw a tantrum when things don't go their way. A teammate who quits mid-game because they're losing, or a student who argues endlessly about one point off their grade, risks getting labeled this way.
The term is usually an insult, pointing out that someone is being overly dramatic about small problems. Everyone gets disappointed sometimes, but calling someone a crybaby can also dismiss real feelings, so it's better to focus on the behavior and what would help.