The Jargon File (version 4.4.7, 29 Dec 2003):

top-post
 n., v.

    [common] To put the newly-added portion of an email or Usenet response
    before the quoted part, as opposed to the more logical sequence of quoted
    portion first with original following. The problem with this practice is
    neatly summed up by the following FAQ entry:

    A: No.
    Q: Should I include quotations after my reply?

    This term is generally used pejoratively with the implication that the
    offending person is a newbie, a Microsoft addict (Microsoft mail tools
    produce a similar format by default), or simply a common-and-garden-variety
    idiot.

    One major problem with top-posting is that people who do it all too
    frequently quote the entire parent message rather than trimming it down to
    those portions relevent to their reply ? this makes threads bulky and
    unnecessarily difficult to read and arouses the righteous ire of
    experienced Internet residents (this style is called ?TOFU? for ?text over,
    fullquote under?, or sometimes ?jeopardy-style quoting?). Another problem
    is that top-posters often word their replies on the assumption that you
    just read the previous message, even though their perversity has put it
    further down the page than you have yet read. Oppose bottom-post.