Initialism, unpronounceable as a word (hence pronounced "ellemjay"), for Loud Mouthed Jackass. An insult, of course, on a somewhat lower order than fugghead and, thus, in the often hyperventilated exchanges in fandom, one which tends to be used far less frequently.
Too, fugghead is generally meant to describe an ongoing state of being, whereas a single specific instance of monumental stupidity goes along with LMJ, according to Dr. Gafia. (And anyone who doesn't agree with this definition is probably either a fugghead or an LMJ, so you can safely ignore them.)
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