This meaning of the word is used exclusively in the AFOL, or Adult Fans Of Lego community. It's a combination of the words clone (as in Star Wars Clone Troopers) and (re)tard.
The term refers to someone who buys a lot of the standard clone trooper figures, and then uses (or rather: abuses) his sharpies while trying (and most of the time failing) to draw cool legion colors on them. They most often adopt online names like "Commander Marks of the ### (fill in any 3 numbers) Legion" and build very shoddy and ugly builds. They react very defensively when somebody calls them a clonetard.
someone who gets really pissed about a lil name calling and is secretly gay (not that there is anything wrong with that) so tries to hide inside someone elses personality, and plus he is retarded
A clontarf is a feminized liberal male douchebag; a know-it-all phloom who has to have the last word in every conversation; a guy who uses puns, big words, and trivia questions to show everyone how smart he is; a dick weed who always corrects your spelling. Like a black hole, he sucks all other conversation out of the room so that he can only hear himself talk.