This is the 13215th most frequent Turkish word.
kaybetmesinin
"The act of his/her/its losing."
Here, 'kaybetmesinin' means 'his losing' and indicates causation by preceding 'nedeni'.
Ali'nin maçı kaybetmesinin nedeni yorgunluğuydu.
The reason for Ali's loss of the match was his fatigue.
In this sentence, 'kaybetmesinin' translates to 'its losing,' referring to the absence of daylight.
Gün ışığını kaybetmesinin ardından hava hızla soğudu.
After it lost the daylight, the air quickly cooled.
Here, 'kaybetmesinin' means 'the act of losing,' used in a context of realization or sentimental expression.