Green’s Dictionary of Slang

(orig. US campus) extremely, very much, primarily.

1978		Columbia Spectator (Freshman Orientation Supplement) 31 Aug. 6: [P]eople getting away with all kinds of incredible garbage, ranging from the merely imbecilic to the majorly zany.
1989		P. Munro Sl. U. 26: My mechanical engineering class is majorly hard.
1993		R. Shell Iced 115: I ran into Mercedes and we proceeded to get majorally fucked-up.
1995		A. Heckerling Clueless [film script] I am majorly, totally, butt-crazy in love with Josh.
2005		P. Howard Curious Incident of the Dog in the Nightdress 6: I feel majorly dizzy.