Negative term for female characters that, aside from having well-endowed bodies, are otherwise utterly pathetic, useless and annoying. Although this term has been informally used in the distant past, it didn't become a meme until trolls on /a/ used it to chide Asahina Mikuru, who is virtually the archetypal moeblob. From there, the term migrated into the rest of the Haruhi fandom, brought there by people who frequent both /a/ and other anime communities, and has since become a common description (no longer an insult in this context) for very "moe" characters. According to Ken Akamatsu, what most know as a "moeblob" is the correct interpretation of what all moe girls in anime should be like.
Presumably, the opposite of this term would be a "Garmass".