What is a moonbat?

According to Samizdata, which claims to have coined the phrase, a barking moonbat is "[s]omeone on the extreme edge of whatever their -ism happens to be". A moonbat would therefore seem to be a milder version of a barking moonbat, in the same way someone who is mad isn't quite as mad as someone who is barking mad. But they're still mad - or moonbats, as the case may be.
Charles Johnson likes calling people moonbat; Crossfire host Paul Begala, for example (http://littlegreenfootballs.com/weblog/?entry=8720_Hope_for_the_Future). Or anyone else whose opinion he disagrees with. The term moonbat produces 642 matches on a site search of LGF. Anyone suggesting Johnson's use of the word is getting slightly inflationary would certainly get slapped down. And called a moonbat.

Barking, if you ask us...