Green’s Dictionary of Slang

gut adj.

[one knows ‘in one’s guts’; gut n. (3)]

1. (US campus) easy.

implied in gut n. (3a)
[US]D. Pinckney High Cotton (1993) 149: That most notorious of ‘gut classes,’ the History of Television.
J.R. Saunders Tightrope Walk 84: The paper hurled epithets such as ‘intellectual farce’ and ‘most outrageous gut course on campus.’.
P. Mackintosh Sneaking Out 81: The ‘C’ In ‘Human Sexuality,’ purported to be a gut course.

2. (orig. US) based on instinct, feeling.

[US]Kerouac Visions of Cody (1973) 42: The powerful gut feeling I had.
[US]G. Swarthout Where the Boys Are 169: Don’t they teach you how to get gut-mad?
[US]B. Moyers Listening to America 92: Wallace appealed to people because he spoke of gut issues.
[UK]C. Dexter Daughters of Cain (1995) 6: I’ve got this gut-feeling that Phillotson wouldn’t have got very far with it anyway.

3. of fundamental importance.

[UK]Economist 17 Oct. 261/3: For Harold Wilson it was a carefully planned campaign: ... the neo-Kennedyism combined with a concentration on gut issues .