Error by the female superlative by overprefijation of acquaintance, which would be "megarchi known", which last, if left too long, separate it with spaces.
It means that it is very but well known or famous a person or something when using two superlatives (mega and archi) to emphasize that relationship of superiority. It can also be said to be known or that it may not be known.
MEGA-ARCHI-KNOWN, A superlative known, taken to one end. Mega is a prefix that means a million, and archi is synonymous with super, so mega magnifies a million times to what is already archi