Where does “megaannum” come from?

megaannum (English) comes from English mega, from Ancient Greek μέγας, from Proto-Indo-European meǵh₂s — great.

Ancestry of “megaannum”, step by step

megaannum traces back through two separate lines of ancestry.

via English mega

StepLanguageWordMeaning
1EnglishmegaVery large; great; excellent
2Ancient Greekμέγαςbig, large; great, mighty, loud; marvelous,...
3Proto-Indo-Europeanmeǵh₂sgreat

via Latin annum

StepLanguageWordMeaning
1Latinannumaccusative singular of annus; year
Every word from Proto-Indo-European meǵh₂sEvery word from Ancient Greek μέγαςEvery word from English mega