For the second appearance in a row C.K. was wearing a suit.
Appearing on the show, host Stephen Colbert asked him if he regretted previously comparing Trump to Hitler, to which Louis replied: "I don't take it back, I regret it - there's a difference". Wasn't even true. Then I said, 'They were liars.'" He then topped his statement off with, "It's just an insane. "I thought he was some new kind of evil but he's just a lying sack of shit".
"Someones people lie", Louis went on. "It was amusing for a little while".
A year ago, C.K. emailed a letter to fans urging them to stop supporting Trump's candidacy.
Tell us what you think!
Fox News' Bill O'Reilly scandal over sex harrassment payouts deepens
In July, then-network president Roger Ailes was forced out after multiple allegations of harassment were lodged against him. The Times reported that O'Reilly and Fox News have reached settlements with five women totaling at least $13 million.
He then described Trump as the type of liar who "likes" to lie. "That's somebody who just they just lie, they like it - he likes it". News. Implying that the President belongs in the highest category of liars, Louis then proceeded to question the validity of Donald Trump's name. "Then you have a liar - nearly like a problem; they can't help it, they lie a lot".
"Then you have a lying sack of s-t", he said to applause from the audience, referring to the president.
Through their blatant Scrooge McDuckery, Green Day have inadvertently helped Stephen Colbert launch a brand new segment on his Late Show.
Press PLAY on the clip above to watch Colbert's O'Reilly-centric monologue in full, then tell us if you agree with his sentiment regarding POTUS' remarks.