This is the 30th most frequent Danish word.
sin
"Sin" is a reflexive pronoun in Danish meaning "his," "her," "its," or "their," referring back to the subject of the sentence.
Here, 'sin' is used as a possessive pronoun referring to an item (the bag) that belongs to the subject (Peter).
Peter tog sin taske og gik.
Peter took his bag and left.
In this sentence, 'sin' refers to the possessive relationship between the subject of the sentence (she) and her younger brother's toy.
Hun fandt sin lillebrors legetøj.
She found her younger brother's toy.
The word 'sin' is employed here to denote possession of the book by the subject of the sentence (the child).