Green’s Dictionary of Slang

butterflies (in one’s stomach) n.

nerves, apprehension, tension.

1948		M. Horowitz ‘Sl. of the Amer. Paratrooper’ in AS XXIII:3/4 319: butterflies. Nervous tension in the pit of the stomach.
1951		R. Leveridge Walk on the Water 319: You’re just jittery because we’re going back up there. The old butterflies.
1962		R. Cook Crust on its Uppers 48: I don’t know how many times I haven’t heard it, but every time it gives me the butterflies.
1974		A. Pierrepoint Executioner 190: I do not believe any man who has done it [i.e. judicial execution] and says he doesn’t get butterflies [...] before the action starts.
1981		M. Baker Nam (1982) 75: It was Hutchinson’s first action. Just as we’re going out he says, ‘Oh, I think I’m sick.’ [...] We didn’t think too much of that shit. It was more like butterflies before a big football game.
1995		P. O’Keeffe Down Cobbled Streets, A Liberties Childhood 55: ‘I’ve got butterflies in my tummy,’ I wailed.
2009		H. Pool Guardian 16 Feb. 🌐 Later, I had butterflies in my stomach as Obama made his way to the podium.