This is the 9373rd most frequent Polish word.
usykowi
"Usykowi" means "to Usyk" (dative case of the name Usyk in Polish).
Here, 'Usykowi' is in the dative case, indicating the indirect object in the sentence, the person to whom the book was given.
Dałem książkę Usykowi.
I gave the book to Usyk.
'Usykowi' is once again in the dative case, used to denote the person being thanked.
Podziękowałem Usykowi za pomoc.
I thanked Usyk for the help.
'Usykowi' in this sentence is in the dative case, representing the entity in whom trust was placed.