Learn Polish: Nieznajomy czeka na mojego przełożonego. - The stranger is waiting for my superior.

Nieznajomy is a substantivized adjective—a word that describes a characteristic but is used as a noun. Even though it translates to a noun in English, in Polish it declines exactly like an adjective. Here it is the subject of the sentence, so it stays in the basic nominative case.

Is przełożonego also an adjective acting as a noun?

Yes! The dictionary form is przełożony (superior or boss). Because it acts as a noun referring to a person but is built like an adjective, it takes standard adjective endings when it changes case.

Why do both mojego and przełożonego take the -ego ending here?

The phrase czekać na (to wait for) requires the accusative case for the object being waited for. Because a superior is a masculine person, its accusative form is identical to the genitive form. That is why the pronoun mój becomes mojego and przełożony becomes przełożonego.

How is nieznajomy related to the verb znać (to know)?

They are part of the same word family. Znać means "to know" a person. From that, we get the substantivized adjective znajomy (an acquaintance, or someone known). Adding the negative prefix nie- creates nieznajomy, meaning "stranger" (someone unknown).