From Longman Dictionary of Contemporary Englishto cap it all (off)to cap it all (off)British English spokenINTRODUCE used before a statement to say that something is the last in a series of annoying, unpleasant, or funny events To cap it all, the phones didn’t work, and there was no hot water. → cap
Examples from the Corpus
to cap it all (off)• I had a terrible day at work, and to cap it all off I got a flat tire.• And to cap it all off, when she was tied-up she couldn't run backwards, so she lay down instead!• And to cap it all she could feel the ominous beginnings of a thundering headache.• And to cap it all, the bland sleazy boredom of it all.• And, to cap it all, Wimbledon won the Cup. 