I thought "costume" means a cloth something funny, like for Halloween. It's not?

Native speaker’s answer
Rebecca
A "costume" can be mean clothes for dressing up, as you mentioned, but it can also mean the clothes you swim in. It has the same meaning as "swimsuit." The meaning of "costume" depends on the context! Ex: Did you bring your costume for the pool? Ex: I didn't get a costume for Halloween. Ex: Rachel bought a new swimming costume for the trip.