This is the 1818th most frequent Russian word.
мужчиной
The word "мужчиной" is the instrumental case of "мужчина," meaning "man" or "a man" in English.
The word 'мужчиной' is used in the instrumental case to indicate the state or role someone has assumed.
Он стал настоящим мужчиной.
He has become a true man.
In this context, 'мужчиной' is also in the instrumental case, describing the person in relation to whom pride is expressed.
Она гордится своим мужчиной.
She is proud of her man.
Here, 'мужчиной' is used in the instrumental case to identify the role portrayed by the subject.