intransitive. To act zealously; to demonstrate zeal; to be zealous for or in support of a cause, etc.

To become eager, animated, or enthusiastic. Also, with up. to warm (up) to: to become interested in, acquire zest for, to ‘put one's back into’…

intransitive. To grow enthusiastic; to behave enthusiastically; to display or express enthusiasm. In later use: to express enjoyment, interest, or…

intransitive. To feel or display enthusiasm or passion.

Excitedly effusive; highly enthusiastic, fervent. Frequently in to wax lyrical. colloquial (originally U.S.).