butterflies in (one's) stomach
A feeling of nervousness. The butterflies in my stomach almost kept me from going on stage and performing.
butterflies in one's stomach
Fluttering sensations caused by a feeling of nervous anticipation. For example, I always get butterflies in my stomach before making a speech. This term likens a nervous feeling to that resulting from swallowing live butterflies that fly about inside one. [c. 1900]