Verb
(idiomatic, sometimes ironic) To finish or complete something; to add a finishing touch.
We went for a walk along the beach, watched the sunset, and then capped it all off with a lovely meal.
What a day. I lost my wallet, my mobile phone broke, then to cap it all I got a flat tyre.
(idiomatic) To surpass or outdo something.
Source: en.wiktionary.org