something that you say when you want to tell someone the worst event in a series of bad events that has happened to you He spilled red wine on the carpet, insulted my mother, and to cap it all, broke my favourite vase.
if you have been describing bad things which happened and then say that to cap it all something else happened, you mean that the final thing was even worse He spilled red wine on the carpet, insulted my mother, and, to cap it all, he broke my favourite vase.