This is the 2468th most frequent Turkish word.
kendinden
"Of oneself" or "from itself."
Here, 'kendinden' means 'of oneself', indicating confidence stemming from the person's own achievements.
O, başarılarından dolayı kendinden emin görünüyor.
He looks confident due to his achievements.
'Kendinden' is used to suggest something originating from the person's character or usual behavior.
Kendinden hiç beklenmeyecek bir davranış sergiledi.
He exhibited behavior that was completely unexpected of him.
In this case, 'kendinden' denotes an inherent characteristic—something built-in or self-contained.