Green’s Dictionary of Slang

extremely, to a great extent.

1966		Current Sl. I:3 5/2: Mega [...] extremely.
1982		Eble Campus Sl. Spring 6: mega- [...] I had a mega-good time.
1990		Eble Campus Sl. Apr.
2000		N. Barlay Crumple Zone 19: If you stay in now you’re just gonna get mega depressed.
2013		K. Sampson Killing Pool 131: This thing has just gone mega-nightmare.