This is the 226th most frequent Azerbaijani word.
su
Water.
The word 'su' is used in its basic meaning as 'water', a drinkable liquid essential for life.
Mən hər səhər bir stəkan su içirəm.
I drink a glass of water every morning.
The word 'su' is part of another word 'suallar', which means 'questions', showing a derivational usage unrelated to its primary meaning.
Bu suallar çətin görünür.
These questions seem difficult.
Here 'su' refers again to its natural physical state as water, with an implication of it affecting the environment (coolness).