It's a more worse version of the word "Retard" usually referring to someone acting like a fool, or doing something stupid. Also means Lame.
Only a regtard would do something so stupid.
A regtard is a Counter-Strike (or any FPS game for that matter) player that plays regularly on a public server and generally makes an ass out of themselves the entire time.
This type of person is prone to typing things like "OH NOES I'M DEAD!!!" and "OMFG soandso's a h4x0r!!!".
More often than not they are under the age of 17 and live in Canada.
"(ADMIN-ONLY) dYn mfspolo: Oh no, Shadow Knight just connected, he's such a friggin regtard."