This is the 2921st most frequent Polish word.
mężem
The Polish word "mężem" means "husband" in the instrumental case.
The word 'mężem' isn't in this sentence; it uses 'mąż', which is the nominative case of the noun.
Mój mąż jest moim najlepszym przyjacielem.
My husband is my best friend.
Again, 'mężem' is not present; 'męża' is used as the accusative case of 'mąż'.
Ona jest dumna ze swojego męża.
She is proud of her husband.
Here, 'mężem' appears as the instrumental case form of 'mąż'.