When You Should and Shouldn't Use It
The acronym AF translates to a swear word, plain and simple. It certainly may be a slightly more respectable option compared to dropping a full blown f-bomb in a tweet or in a text message, but unlike other Internet slang terms like LOL and BRB, this is a term that you should avoid using in some situations.
Refrain from using this term in professional situations or when messaging people you don't have as much of a casual and carefree relationship with. If you wouldn't say it out loud in person, then don't say it online or via text message either.
A more proper way to communicate the same thing would be to use words like very, really, or extremely before the adjective.
In some strange way, however, using this acronym online is at least slightly more polite than writing it out as a full F-bomb in a similar way that people have been using "WTF" (What The F***) for years.
It's up to you to determine when it is or isn't appropriate to use it according to the conversation and who you're speaking to online or via text message.
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So, when should you use it? Use it whenever you want -- on Twitter, Facebook, Instagram, text message -- in the most laidback or even humorous situations and conversations. It just may be wise to avoid using it as much as possible when writing an email to your boss