Its not an abbreviation. The term originated circa 1200 when the internets were still young. In order to post a long paragraph on a forum, you had to let a priest kick you square in the nuts, because it was thought of as sinning (don't ask why). So occasionaly, someone would write at the end of the long post that it "hurt" (everyone knew what they meant because it was a common practice). Eventualy, some smartass realized that right after one was kicked in the bollocks, saying hurt out loud would sound more like "hrth", so this new term was adopted. The term has lost all its meaning nowadays.
hrth!