This is the 8363rd most frequent Turkish word.
nesin
"What are you?"
The word 'nesin' here refers to the second person singular 'you' combined with the verb 'to be'.
Nesin bu konuda gerçekten çok başarılı bir yazardır.
You are a very successful author in this field.
The word 'nesin' here means 'what do you have', asking for the item being held.
Elinde nesin var, onu herkes görmek istiyor.
What do you have in your hand? Everyone wants to see it.
The word 'nesin' here is asking for clarification about the thing causing fear.