This is the 9343rd most frequent Polish word.
ukraińcem
"Ukraińcem" means "Ukrainian" (male or referring to a Ukrainian person) in English.
The word "Ukraińcem" is used here as a masculine noun in the instrumental case, following the verb "jest" to describe someone's nationality.
Mój kolega jest Ukraińcem.
My friend is Ukrainian.
"Ukraińcem" appears as a noun in the instrumental case, used to express the state of being associated with a particular identity.
Jest dumny z tego, że jest Ukraińcem.
He is proud of being Ukrainian.
In this sentence, "Ukraińcem" is a masculine noun in instrumental case, used in a relative clause to describe the subject.