The term HODL was accidentally coined in 2013 by a user named GameKyuubi on the Bitcointalk forum.

GameKyuubi posted a message on December 18th with the title: “I AM HODLING”. He was suspected to have made a typo, but he kept writing “HODL” in his posts. His statements went viral and were made into hundreds of memes. Now, the word HODL has forever entered the vocabulary of Bitcoiners. "Never sell your Bitcoins"

Question: When can I sell my Bitcoins?

Answer: No. You don't sell your Bitcoins.

Question: Why shouldn't I sell my Bitcoins?

Answer: Bitcoin is money, much better than the money you know. So, hodling and spending on real products and services, is what you do.