The Brief: FOMO is short for "fear of missing out" and refers to the feeling someone might get when they don't really want to do something, but they're afraid they'll regret passing up the opportunity.

DEEP DIVE
FOMO is short for “fear of missing out” and refers to the feeling someone might get when they don’t really want to do something, but they’re afraid they’ll regret passing up the opportunity. The term can also refer to a feeling of regret or being left out that someone feels when other people are at an event they wanted to go to. Social media can exacerbate FOMO as it is easier to see posts and pictures of what someone missed out on.
https://twitter.com/DanaSchwartzzz/status/1020505156549406722
https://twitter.com/ashleyophoto/status/1020413192131039235
If you can get through fomo you can get through just about anything. Sitting at home while all your friends are at a festival is by far the worst feeling on the planet
— Bassneckhair π€ (@JordanKarlik) July 15, 2018