The story
From a catchphrase said repeatedly by the character Ygritte to Jon Snow in the novel A Storm of Swords (2000) by George R. R. Martin. The phrase, initially used reproachfully and then later affectionately after Jon and Ygritte fall in love, was popularised by its use in the television adaptation Game of Thrones.
Etymology from Wiktionary, CC BY-SA 4.0.