Green’s Dictionary of Slang

(orig. US) very important, the most powerful, highly impressive.

1941		Brackett & Wilder Ball of Fire [film script] That’s a major-league [engagement] ring!
1951		New Yorker 17 Nov. 71: Four pages of orchestration an hour [...] is considered major-league speed .
1972		D. Jenkins Semi-Tough 222: [She] just stood there with her arms at her side [...] displaying her major league lungs. And looking casual.
1986		C. Stroud Close Pursuit (1988) 54: Major league with a snake-line straight into Cartagena or just a small-time banker in the back of the poolroom.
1996		‘Iceberg Slim’ Trick Baby [blurb] Robert Beck, who used Iceberg Slim as his moniker, was a major league pimp who enjoyed serious success in Chicago.
2006		C. Hiaasen Nature Girl 157: Those are some major-league boobs.
2020		(con. 1991-94) W. Boyle City of Margins 192: ‘This is your girlfriend [...] She’s major league’.