know one's stuff

verb

know one's stuff

verb

1

Informal To be knowledgeable about and competent at a particular subject or task.

Joe really knows his stuff when it comes to baseball.
The field of knowledge in which Hori claimed to be an authority was Palestinology. The contents of the missive show that Hori certainly knew his stuff.2020, Emmet Sweeney, The Theban Empire, volume 3, page 176:

Synonyms: know ball, know one's onions, know one's shit, know one's way around, know the drill, know the score, know which end is up
Antonyms: know one's ass from a hole in the ground

Entry derived from the Wiktionary, under licence CC BY-SA 4.0 — list of authors.