"First Post!" is a phenomenon of Internet discussion groups (notably Slashdot and LiveJournal), where participants strive to be the first person to add a comment ("post") to a new article or discussion thread. The phenomenon is largely confined to sites that have reached a high degree of popularity, such that users are genuinely surprised to see an article without any associated comments. There is also the necessary condition that comments are displayed in chronological order (meaning the first message is the most prominently displayed). It is prevalent among user-commentary sites and does not often appear on conventional webboards, community journals, etc.
Originally, a first post was a light-hearted expression of the poster's excitement at being the first person to write a comment: "OMG I got 'First Post!'". However, it is now more widely intended as a means of annoying other site users (see Internet troll), with the aim of provoking a negative reaction (see Flamebait).
Since the chance of achieving a first post is increased by making the comment quickly, the message is usually short, adding no insight to the discussion. Often, it only contains the text "First post!" (or variations thereof). Some people would even argue that a comment is only a true "first post" if it contains no relevant information.
It should be noted that the traditional text of a first post may vary according to the site's content. For example, on Blog for America, "44!" is used to represent a first post, in reference to the 44th presidency.
Some forums have devised creative ways of eliminating messages of this type: the filters on Fark.com message boards automatically turn the words "First Post!" into the word "boobies" and timestamp the comment 12 hours after it was actually posted. Meanwhile, Something Awful made posting "First Post!" a bannable offense. One way users have subverted this rule is to adopt a similar but different message (most notably "Frost Pist").
On political blogs (such as Atrios), "first post" is often typed as "Frist," a reference to Senate Majority Leader Bill Frist (and depending on the forum, as a way to add an extra dash of irritation to an already irritating practice).