I was a little stumped on this one, I couldn't figure it out despite
many wasted hours playing Counter Stike, and even more wasted years on
IRC. However, it became obvious when I read austin_trill's comment
below, and did a little more searching.
The term is derived from Chris Coutt's Romeo and Juliet - a l33t
translation of Shakespear's Romeo and Juliet:
It is the second line delivered in the first act, and it meant to
paraphrase the conversation between Gregory and Sampson, and is a
derogatory reference to Sampson by Gregory and could be expanded as:
Shut the f*** up, Sampson, you fag. It should be noted that fag,
used in a l33t sense, means literally 'homosexual', although is not
necessarily indicative of any real homophobic feelings beyond the
acceptance of the term as an insult.
Therefore the expression STFU Sampfag is more or less a literary
quote, rather than an actual word. It has no real meaning beyond it's
existence as a quote from this Flash movie, which is widely known
within the l33t community.
However, the term now seems to have entered common usage as a
generally insulting term for a dumb or boasting person. This is
similar to the way people use character names from popular tales to
describe people (eg. Don Juan, Casanova, Forrest) it is used to
exemplify characteristics in it's targets that are taken from the
context of it's namesake.
For examples of this, see the following pages:
(Some mature language is likely to be found in the following links)
There are a few other examples that I will not link to, simply because
of excessive difficulty in really reading them, or because they are
surrounded by unrelated things, but you get the idea.
The actual passage it is paraphrasing is this:
The quarrel is between our masters and us their men.
'Tis all one, I will show myself a tyrant:
when I have fought with the men I will be cruel with the maids,
I will cut off their heads.
The heads of the maids?
Ay, the heads of the maids, or their maidenheads;
take it in what sense thou wilt.
They must take it in sense that feel it.
Me they shall feel while I am able to stand:
and 'tis known I am a pretty piece of flesh.
'Tis well thou art not fish; if thou hadst,
thou hadst been poor-John.--Draw thy tool;
Here comes two of the house of Montagues.
(Source: http://www.ibiblio.org/gutenberg/etext98/2ws1610.txt Romeo
and Juliet from Project Gutenberg)
I hope this answers your question, and I thank you for giving me an
excusing to watch that Flash movie again. :)