Wikipedia:
HODL is a slang term and Internet meme that is used in the Bitcoin community when referring to keeping, i.e. holding onto the cryptocurrency rather than selling it. It originated in a December 2013 post on the Bitcoin Forum message board by an apparently inebriated user who posted with a typo in the subject, "I AM HODLING". In 2017, Quartz listed it as one of the essential slang terms in Bitcoin culture, and described it as a stance, "to stay invested in bitcoin and not to capitulate in the face of plunging prices". The Street.com referred to it as the "favorite mantra" of Bitcoin holders. According to the Financial Times, it was the reddit forum Buttcoin that helped popularize its use.

Since entering the Bitcoin vernacular, HODL has become a backronym for "Hold On for Dear Life".