This is the 4985th most frequent Russian word.
купальнике
Swimsuit.
The word 'купальнике' is used in the prepositional case to indicate the location or state of wearing a swimsuit.
Она загорала на пляже в красном купальнике.
She was sunbathing on the beach in a red swimsuit.
Here, we use 'купальник' in the nominative case as the object being received.
Моя сестра получила подарок — новый купальник.
My sister received a gift — a new swimsuit.
In this sentence, 'купальник' is used in the accusative case as the object of the verb 'надеть'.