An expression that can be added to almost any adjective to give emphasis. It generally means, "Extremely" or "To the utmost degree." This expression is very flexible (you might say it's flexible as fuck?), as you can use it to modify essentially any description, good, bad or other.
Opinions are divided as to what the word 'Fuck' means in this context:
One one hand, you could easily argue that it is referring to the act of 'Fucking
.' IE "This thing was thingy as fuck," meaning that the thing was so thingy that it resembled fucking. This was probably where this phrase originated.
However, as mentioned above, the phrase can also be used alongside negative descriptions as well as positive ones. IE "That shit was painful as fuck!" Given this, it's safe to assume that the word fuck, in this context, has no meaning at all, and is likely just used for emphasis, as it is a curse word.
The phrase can be used by anyone, but is limited in reach by it containing the word 'fuck.' The phrase is also somewhat vacuous, and more-or-less accomplishes the same thing as an exclamation point. Due to its loud and vapid nature, it has become very popular in the Bro
-This is legit as fuck! (probably the most common usage)
-My hand-writing is girly as fuck
-This sandwich is dank
-I am writing this UD post because I'm bored as fuck
-Dis club is tight as fuck
Atypical but still appropriate uses:
Programmer: "This algorithm is optimized as fuck!"
Doctor: "That patient's sick as fuck!"
Hipster: "This restaurant is main-stream as fuck!"