to feel very happy and excited (especially when sth. Good has happened)

e.g. Jenny is walking on air because her friend invited her to the school dance. She can't wait to go.

an unknown person who enters a race and gets a good grade

e.g. Did you hear about the dark horse in the running race yesterday? It was his first race and he finished in the top ten.

to act quickly, without thinking

e.g. I hate working with Mary. She's quick on the trigger and always ready to get mad.

to encourage sb. to do sth. (especially sth. They shouldn't do)

e.g. When your classmate eggs you on to copy his test paper, you should say “no” to him.