From Liu Yiqing's Southern Dynasties
世说新语·
规箴:
高足之徒，皆肃然增敬 - describing how advanced disciples listening to a lecture 'all grew solemn and more respectful.' The phrase hardened into
肃然起敬 in later literature.
肃 is 'solemn, reverent'; the
然 makes it adverbial.
