This is the 16439th most frequent Polish word.
wegetarianinem
"Wegetarianinem" means "vegetarian" (instrumental case, masculine form) in Polish.
In this sentence, 'wegetarianinem' is the instrumental case form of 'wegetarianin' used to indicate the noun complement of the verb 'to be' (jestem).
Jestem wegetarianinem.
I am a vegetarian.
Here, 'wegetarianinem' is again in the instrumental case, used after the verb 'zostać' (to become) to indicate transformation into the specified role or state.
Został wegetarianinem, aby poprawić swoje zdrowie.
He became a vegetarian to improve his health.
In this context, 'wegetarianinem' appears in the instrumental case while explaining the state of being a vegetarian in relation to certain actions such as changing habits.