To have butterflies in your stomach

Example: “Right before walking on stage, she had butterflies in her stomach.”

Origin: Names a genuine physical sensation rather than a purely poetic one. Under stress the body's fight-or-flight response redirects blood away from the digestive system and toward the muscles, and the resulting change in the gut produces exactly the light, fluttering, hollow feeling the phrase describes — which is why the sensation is so consistent from person to person and so instantly recognizable. The earliest citation usually given in English dates to 1908, in Florence Converse's novel "The House of Prayer," though the comparison is natural enough that it may well have circulated in speech before anyone thought to write it down. It spread widely through both American and British English over the middle of the twentieth century, attaching particularly to stage fright and to nerves before an examination or a performance.