a world of good
A very positive effect (on someone or something). If you're feeling sad, getting out in the sunlight should do you a world of good.
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]
world of good, a
An enormous benefit. World has been used to mean “a great deal” since the sixteenth century, but this phrase dates from the nineteenth century and was generally used in connection with something that was beneficial to one’s health. “The mountain air will do him a world of good,” wrote Thomas Mann in The Magic Mountain (translated by H. Lowe-Porter, 1927).