This is used when you need a stronger word to ask someone to shut up.

In fact, the acronym has been changed by the years of usage and is now, a word by itself.

***The word "STFU" is said "STUFU".

So, "Stfu", spelled "Stufu" is said instead of "Shut Up".

But some exceptions occurs when it is employed as a single word instead of a sentence.

For example ; if you say "Stfu" only, it is a sentence meaning "Shut up", but if you say "Shut the stfu", "Stfu" becomes just a word, combined with "shut the", meaning anything stronger than "(shut the) fuck up" or "(shut) up", giving a stronger impact on the sentence.

When you eat dirt for the first time and leave your family to build a shack in the woods