Crossword Genius

Thereby, one understanding nothing must keep quiet mostly (4,5)

Ross

I believe the answer is:

ipso facto

'thereby' is the definition.
(Latin phrase meaning thereby or by that very fact)

I cannot understand how the rest of the clue works.

(Other definitions for ipso facto that I've seen before include "Thereby (Latin)" , "By that very act (Lat.)" , "By that truth (Latin)" , "Self-evidently; thereby" , "For that precise reason" .)

I've seen this clue in The Times.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.