Learn Hardcore Icelandic: Sundfötin hennar þornuðu á stólnum meðan við gengum á sandinum. - Her swimwear dried on the chair while we walked on the sand.

Even if English might use a singular idea like her swimsuit, Icelandic often uses the plural noun sundföt. That is why the verb is also plural later in the sentence.

The ending -in is the definite article attached to the noun, so sundfötin means the swimwear or her swimsuit depending on context.

Why is it sundfötin hennar and not something like hennar sundfötin?

In Icelandic, possessives like hennar are very often placed after the noun.

So:

sundfötin hennar = her swimwear
literally: the swimwear her

This is a very normal Icelandic pattern, especially with pronoun possessives:

maðurinn minn = my husband
bíllinn hans = his car
húsið okkar = our house

So although it feels reversed from English, sundfötin hennar is exactly the natural Icelandic structure.