butterflies in his stomach

Butterflies in his (or her) stomach is the expression used when you feel a feeling of nerves and agitation. Typically when someone is nervous, their heartbeat reacts by pounding faster and harder, similar to heart palpitations. These heart palpitations and nervous stomach cause you to feel flutters inside your stomach which feel very similar to butterflies. For this reason, the feeling of a nervous stomach is also called butterflies in the stomach.