In good order or good health, satisfactory, as in He was very ill, but he's right as rain now, or If she'd only worked on it another week everything would have been as right as rain. The allusion in this simile is unclear, but it originated in Britain, where rainy weather is a normal fact of life, and indeed W.L. Phelps wrote, “The expression 'right as rain' must have been invented by an Englishman.” It was first recorded in 1894.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.