In a classic case of the Baader-Meinhof phenomenon, I recently read "The Truth" by Terry Pratchett, which repeatedly makes reference to that phrase, and I am now noticing it everywhere, whereas previously I can't recall being aware of it at all.

I remember this from the Stephen Colbert show where he then goes something like “what was the truth doing with its pants off”

It’s a very old saying but we all learn it at some point

Quote Investigator was quite interesting on the history. The phrase has been evolving over many years going back to at least 1710. https://quoteinvestigator.com/2014/07/13/truth/

"As Mark Twain famously never said" (c)