Although the quote first appeared in the author J.R.R. Martin’s book A Storm of Swords, the quote is most recognized from the series’ season 2, episode 7.
In this episode, protagonist Jon Snow is accompanied by wildling character Ygritte. During one scene they listen to a ballad that sings about giants. The song touches Ygritte.
Jon tries to comfort her but Ygritte finds it only offensive and says “You know nothing, Jon Snow!”
The series had the quote reappear throughout the seasons on multiple occasions.
Fans of the Game of Thrones franchise quickly started to recognize the quote as a classic catchphrase, after the episode aired in 2012.
You know nothing, Jon Snow inspired many to create both fan-arts and image macros on sites like Deviantart, Twitter, Reddit or Quickmeme.