"Moonbat is a political epithet popularized in 2002 by Perry de Havilland of Samizdata, a libertarian weblog. Contrary to some speculation, de Havilland has stated it was not originally a play on the last name of George Monbiot, a columnist for The Guardian, regardless of the fact he and Mr. Monbiot have appeared on the BBC together expressing politically opposed views. The term was originally rendered as 'Barking Moonbat', suggesting that certain issues seem to trigger a reflexive response from some people much like wolves howl at the moon (i.e. the term evokes the traditional association between the moon and insanity). It now enjoys great currency in the conservative and libertarian blogosphere as a label for certain individuals of the American radical Left, often used to criticize their opposition to the Iraq War. "Moonbat" similar to the epithet Idiotarian and like that term can also be applied to people anywhere on the political spectrum (for example both terms have been used to describe US conservative Pat Buchanan).
According to de Havilland, a moonbat is "someone on the extreme edge of whatever their -ism happens to be". Adriana Cronin defines the term as "someone who sacrifices sanity for the sake of consistency". This term has long been used to describe protesters on the political Left, but was originally coined to also describe commentators on the political Right as well as certain libertarians."
I learn something new every day at FR. :-)