Nash Grier -- the most followed person on Vine -- billowed out a homophobic slur so loud and angry it is truly shocking.

The 16-year-old posted the video in April -- then quickly deleted it.  It features a snippet from a TV commercial about HIV.  Grier clearly took issue with the message.

The reason it just surfaced -- Tyler Oakley, another social media star, got his hands on it and was appalled enough to blast it out.

Grier has since taken to Twitter to apologize.