This is the 12830th most frequent Russian word.
купальником
The Russian word "купальником" translates to "swimsuit" or "bathing suit" (in instrumental case) in English.
The word купальником is used here in the instrumental case, indicating it is used in association with another action (with a new swimsuit).
Она идет на пляж с новым купальником.
She is going to the beach with a new swimsuit.
Here, купальник is in its nominative form, used as the direct object of the sentence (the swimsuit is being referred to as the item chosen).
Ты как выбрала этот купальник?
How did you choose this swimsuit?
In this sentence, купальнике is in the prepositional case, which is used after certain prepositions indicating location or state (dreaming about a swimsuit).