What does "うらめしや" means?

よ~、ビニです。

There is a song by hoshino gen called 異世界混合舞踏会 (or "I wanna be your ghost")

It is a song related to ghosts as you can see.

In the lyrics, the word "うらめしや" comes up.

The only meaning to this word i could find on internet was on "goo辞書" which says: 

うらめし‐や【恨めしや】

幽霊が出てくるときに言うとされる言葉

Although, I got a lil bit confused bout this explanation. Is it the sound that a ghost vocalizes to the living ones?

If it is, is it equivalent as the onomatopoeia "boo"? Any idea where this word come from?

Thanks in advance.

Yep, it is a word that is said by ghosts when they appear.

It comes from the word 恨(うら)め(め)し(し)い(い), which is an i-adjective which the dictionary lists as "reproachful; hateful; bitter"

It makes more sense when you look at the verb 恨(うら)む(む), which has the meaning "to bear a grudge against; to resent; to blame; to curse; to feel bitter towards"

Ghosts in traditional Japanese stories often bear a grudge against someone, and because of that grudge remain in the physical world. So when a ghost appears and says うらめしや, it means something like "I resent you", or "I have a grudge against you".

You can read a bit more detailed explanation here (Japanese link).

Couldnt have any other better explanation. Thanks flint