world of good, a
A great benefit, as in A vacation will do you a world of good. This expression uses world in the sense of “a great deal,” a usage dating from the 1400s. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.