To finish or complete an experience or situation in an especially positive or negative way. Our date was fantastic! We went for dinner, saw a great film, and then we capped it off with some salsa dancing! What a bad day: I left my phone in the grocery store, my car broke down on the way home, and, to cap it all off, my groceries broke through the bag as I was walking to the front door.

A phrase that precedes the last item of whatever the speaker is listing. I was late to school, I failed my test, and, cap it all off, I got into a car accident on the way home! To cap it all off, I think you should add some color to the last slide in your presentation.