To talk about everything from astronomy above to geography below; to chat about all sorts of things with a wide range of knowledge.

to chat idly about all sorts of things; to talk endlessly about everything under the sun

to talk about everything under the sun; to chat idly; to be eloquent or skilled in speech

sobriquet of Zou Yan, a Warring States period philosopher of the Yin and Yang school, named for his grand and expansive discourse on the heavens