This is the 9584th most frequent Polish word.
wujek
Uncle (maternal or paternal).
Here, "wujek" is used to denote a familial relationship, specifically one's uncle.
Mój wujek przyjechał wczoraj z Zakopanego.
My uncle arrived yesterday from Zakopane.
In this sentence, "wujek" is used informally to address a clown, showing a friendly and child-appropriate ay of addressing them.
Na urodzinach dzieci miały okazję poznać wujka klauna.
At the birthday party, the children had a chance to meet Uncle Clown.
Here, "wujek" is used humorously or respectfully as a nickname for an elder colleague or someone relatable.