It is a perjorative term... taken from the word "retarted". A "retard" is someone who is mentally disabled. An American "leftist liberal" (not to be confused with an actual "classical liberal" like Thomas Jefferson) has a very dogmatic and mean spirited narrative.

Like mentally retarded people, they are hopelessly trapped in their condition.

The logic they use to support their tenets, when turned on themselves, usually debunk their own beliefs. They do not see their logically conflicted world view (like supporting biological Darwinism, but not social Darwinism) and become inordinately upset, usually resorting to name calling, when someone points these issues out.

Thus, they are Libtards.