The Brief: NSFL stands for "not safe for life."
Compared to NSFW, NSFL content is more triggering or disturbing. Engaging with it is not for the faint of heart, and the use of the acronym acts as a viewer discretionary warning or a trigger warning.
Of course, the phrase also has some comedic uses. Some users tag a post as NSFL sarcastically, as a comment on how not-triggering a photo or a story is. Someone else might use the acronym to share good news, as if saying “Are you sitting down?”
This is NSFL
— OK FOX 🆗🦊 (@0KF0X) February 19, 2019
Hey. I have something important to say. Some information you may not be aware of. It's probably NSFL, especially if you're not from around here. You've been warned.
If you're bold enough, read on…
— Ben Brode (@bbrode) February 23, 2019