Where does “Mega-” come from?

Mega- (Portuguese) comes from Ancient Greek μέγας, from Proto-Indo-European meǵh₂s — great.

Ancestry of “Mega-”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	μέγας	big, large; great, mighty, loud; marvelous,...
2	Proto-Indo-European	meǵh₂s	great

Mega- — Etymology, Origin & Word History | Etymology Explorer