What is the origin of the idiom "have butterflies in one's stomach" and when to use it?

The idiom "have butterflies in one's stomach" is believed to have originated from the physical sensations that people experience when they are anxious or nervous. The fluttering feeling in the stomach, similar to the fluttering of butterfly wings, is commonly associated with heightened emotions and anticipation. This phrase is used to convey a sense of nervous anticipation, acknowledging the emotional and physical reactions that can accompany important events or situations.