The story
Referencing a scene from the American comedy film A Very Brady Sequel (1996), in which the character Jan Brady (Jennifer Elise Cox) is lying about having a boyfriend named George Glass at school, to which her sister Marcia Brady (Christine Taylor) replies, “Sure, Jan.” The phrase was popularized in early 2015 on Tumblr and other websites, chiefly through image macros and GIFs featuring the original scene from the film.
Etymology from Wiktionary, CC BY-SA 4.0.