This is the 2464th most frequent German word.
seinen
"Seinen" is the third-person singular possessive pronoun in German, meaning "his" or "its" for masculine or neuter nouns in the accusative or dative case.
'Seinen' in this example is the accusative singular masculine form of 'sein' (his), directly referring to 'Hund', which serves as the direct object of the action.
Er gab seinen Hund mit großem Bedauern ab.
He gave up his dog with great regret.
Here, 'seinen' is the dative plural form of 'sein' (his), agreeing with 'Freunden' as the indirect object of the sentence.
Er zeigte seinen Freunden das neue Buch.
He showed his friends the new book.
In this sentence, 'seinen' is the accusative singular masculine form of 'sein' (his), modifying 'Vorschlag', which is the direct object.