cap it all

1

Also, cap it all off. Finish or complete something, as in To cap it all off they served three kinds of dessert.

2

Surpass or outdo something, as in This last story of Henry's caps them all. Both usages employ cap in the sense of “topping” something. [First half of 1800s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.