This is the 1853rd most frequent Japanese word.
曰く
"曰く" means "according to" or "as stated by."
曰く is used here to mean 'according to [someone]', expressing someone's quoted statement.
彼曰く、今日は忙しいので会えないそうです。
According to him, he is busy today and cannot meet.
曰く is utilized here to indicate her point of view or statement about the restaurant's food.
彼女曰く、このレストランはとても美味しいらしい。
She said that this restaurant is very delicious.
曰く is used in this context to denote information or findings according to his research.