Not actually a word. It's the sound of a girl screaming.
You want an example? Put a fake spider in your sister's bed.
An onomatopoeia refering to a scream. Popularised in Manga and Anime, it is often associated with fangirls who are notorious for high pitched screams that cause permanent ear canal damage to everyone within a 2 mile radius.
"OMG! It's Zac Efron! KYAA!"