"Sure, Jan" is used when you can easily read a family or friend for the filthy liar they are. You say it when you obviously know they are lying but you don't want to call them out in front of everyone. Phrase originated from A Very Brady Sequel.
Marcia- "That's funny, I've never heard of a George Glass at our skül"
Jan- "That's because he's a transfer student. He came in the last week of school. He's really good looking and thinks I'm really cool."
Marcia- "Sure, Jan."
by Marcia Glass January 27, 2015
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A phrase coming from the TV show known as the Brady Bunch, in which Jan tells her family about her fake boyfriend George Glass and Marcia replies "Sure Jan"
Marcia- "That's funny, I've never heard of a George Glass at our skül"

Jan- "That's because he's a transfer student. He came in the last week of school. He's really good looking and thinks I'm really cool."

Marcia- "Sure Jan"
by paenacali February 4, 2015
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A sentence used to sarcastically dismiss another person's obviously fictitious story.

It originated in the 1995 movie "The Brady Bunch", a parody take on the TV series of the same name.

In one of the earlier scenes of the movie, Jan, one of the Brady children, wants to impress her family and makes up an obviously completely made-up story about having started dating a boy named George Glass at her school, every time someone in her family inquires about this boy, she keeps piling on the lies up until her older sister Marcia ends the conversation with a curt "Sure, Jan." faking agreement but clearly being so done with Jan's crap.
Donald Trump:
"The aid my administration brought to Puerto Rico was a complete success, we did better than any other administration before!"

Everybody else:
"Sure, Jan."
by MapDark-Dark September 14, 2018
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