An unthinking or insane
leftist -- in other words, most modern leftists.
Moonbat can also be used as an adjective, e.g. a moonbat professor. According to the
Wikipedia entry for moonbat, the word was coined in 2002 by the Editor of Samizdata,
Perry de Havilland, and was a variation on the name of
radical British activist and columnist George Monbiot.
Originally, the term "moonbat" was intended to be more politically neutral, and described wackos on the left and the right, but it quickly acquired its current usage of being applied almost exclusively to those on the left.
The term also references the moon much in the same way that "lunatic" refers to the insanity-causing powers of the full moon (
luna = moon). Bloggers occasionally analyze the behavior patterns of various moonbat "species" as if they were actual animals, and even give them satirical Linnaean taxonomical names, such as "moonbattus berkeleyensis".
The entry in the Samizdata glossary indicates that
Perry originally coined the full phrase "barking moonbat"; apparently "moonbat" is just a subsequent shortened version of "barking moonbat," rather than being a pre-existing term that was lengthened to barking moonbat.
Worker #1 "Did you see those moonbats on
the news staging another one of their
useless protests?"
Worker #2 "Yeah, all
eight of them."
Worker #1 "Man, they need to get real jobs."