The Brief: NSFL stands for "not safe for life."

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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?”