Green’s Dictionary of Slang

mega adv.

extremely, to a great extent.

[US]Current Sl. I:3 5/2: Mega [...] extremely.
[US]Eble Campus Sl. Spring 6: mega- [...] I had a mega-good time.
[US]Eble Campus Sl. Apr.
[UK]N. Barlay Crumple Zone 19: If you stay in now you’re just gonna get mega depressed.
[US]M. Lacher On the Bro’d 229: [We] blew past this car mega-fast.
[UK]K. Sampson Killing Pool 131: This thing has just gone mega-nightmare.