It's when you behave online like you are 15 years old. Everything about you has to scream 'enlightened contrarian' or 'devil's advocate' otherwise you'll become one of those conformists clicking their fingers to Taylor Swift and Imagine Dragons.

Conversations with them are impossible and frustrating because they aren't looking to talk to someone, they're looking to talk AT someone and collect as many upvotes as possible.

Urban dictionary definition - https://www.urbandictionary.com/define.php?term=edgelord

But in general people who try to be "edgy" and "dark" to a fairly ridiculous degree.

Imagine someone who makes an edgy joke, then imagine someone who makes only edgy jokes, constantly - then they stop being funny.

obligatory xkcd : https://xkcd.com/2036/