An exasperating individual, as in He was only five, but he was a holy terror, running wild through the house and throwing whatever he could lay his hands on. The adjective holy here is an intensifier. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.