have butterflies in (one's) stomach
To have a feeling of nervousness, often before a performance or undertaking of some kind. I almost didn't go on stage and perform tonight because I had butterflies in my stomach.
have butterflies in your stomach
have a queasy feeling because you are nervous. informalget/have ˈbutterflies (in your stomach)
(informal) get/have a nervous feeling in your stomach before doing something: I always get butterflies (in my stomach) before an interview.