sucker
A person who is easily fooled or tricked by others.
A sucker is someone who's easily fooled or tricked, often because they're too trusting or don't think carefully before acting. When a con artist tells an elaborate lie and someone believes it without question, that person is being a sucker. If your older sibling convinces you that “gullible” isn't in the dictionary and you actually go check, they might laugh and call you a sucker.
The word often appears in the old saying “there's a sucker born every minute,” meaning there will always be people ready to believe things that seem too good to be true. Someone selling a “magic” rock that grants wishes is looking for suckers who'll buy it. A student who trades their new bike for a broken skateboard because someone promised it was “really valuable” got played for a sucker.
Being called a sucker stings because it means you let yourself be taken advantage of. People sometimes use the word for someone who ignores warning signs or believes things that don't make sense because they want an easy reward. It's also a word people use to be mean, so it's better to focus on learning from mistakes than on name-calling.
The word also has an unrelated meaning: a hard candy on a stick, like a lollipop. You might ask for a cherry sucker at the store.