This is the 3922nd most frequent Polish word.
siergiej
"Siergiej" is a Polish given name equivalent to "Sergei," commonly used in Eastern Europe.
Here, 'Siergiej' is used as a proper noun, specifically as a first name, to reference a person.
Siergiej jest moim przyjacielem.
Siergiej is my friend.
In this sentence, 'Siergieja' is the genitive form of the name 'Siergiej,' indicating possession, which reflects the context of the event celebrated.
Na imieninach Siergieja było bardzo wesoło.
At Siergiej's name day, it was very cheerful.
Here, 'Siergieja' is again in the genitive case due to the grammatical structure after the verb for 'to meet,' indicating the person being introduced.