Meaning of Johnny-on-the-spot

John•ny-on-the-spot

Pronunciation: ( jon'ē-on-u-spot', -ôn-), [key]
— n. Informal.
  1. a person who is on hand to perform a service, seize an opportunity, deal with an emergency, etc.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.