This is the 3276th most frequent Polish word.
szyi
"Szyi" means "of the neck" in English.
Here, "szyi" is in the genitive case, following "ból" to indicate possession or relatedness (pain of the neck).
Ból szyi jest często spowodowany przez niewłaściwą postawę.
Neck pain is often caused by poor posture.
In this sentence, "szyję" is used in the accusative case due to the directionality implied by the verb "założyć."
Założyłem naszyjnik na szyję.
I put a necklace on my neck.
"Szyi" appears again in the genitive case, showing possession (your neck).