This is the 53rd most frequent Azerbaijani word.
çünki
"Because."
The word 'çünki' is used as a conjunction here, showing a cause-effect relationship.
Mən evdə qalıram, çünki hava yağışlıdır.
I am staying at home because the weather is rainy.
The word 'çünki' introduces the reason for the following statement.
Çünki o çox yoruldu, ona kömək etmək qərarına gəldim.
Because he was very tired, I decided to help him.
'Çünki' here is employed to provide the rationale for the preceding event.