I've always thought that a funny approach (even tho it's most likely wrong, and I dont care :P) was that the m stood for "my" making the program pronounced "my are sea"
'em eye are sea' 'em eye are see' 'mega' 'micro' (2.1a is rather small/simple if you ever try it (do note that it will crash when you join channels because some networks/IRCd's don't a ":" in the /names, this also goes for really old copies of mIRC))