This is the 563rd most frequent Japanese word.
全然
"全然" means "entirely" or "completely," often used with a negative to mean "not at all."
The word '全然' is used here in a negative sentence to emphasize the degree of 'not being interesting'.
この本は全然面白くない。
This book is not interesting at all.
In this sentence, '全然' emphasizes the lack of heat, paired with a negative conjugation of 'hot'.
今日は全然暑くない。
Today is not hot at all.
Here, '全然' is used positively to mean 'completely' or 'totally,' which can occur although it is less common.