This is the 4242nd most frequent Turkish word.
partisini
"Partisini" means "his/her/its party" in English.
Here, 'partisini' refers to 'her party,' showing possession and the object of the action.
O, doğum günü partisini iptal etti.
She canceled her birthday party.
In this sentence, 'partisini' indicates 'her party' as the event being planned by her, again denoting possession.
Arkadaşlarıyla partisini planlarken çok heyecanlandı.
She got very excited while planning her party with her friends.
Here, 'partisini' refers to 'her party' as the event being organized, showing possession of the object.