1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): *right as rain
Cliché perfectly fine; all right. (Based on the alliteration with r. *Also: as ~.) • Lily has sprained her ankle, but after a few weeks of rest she should be as right as rain. • All we need to do is tidy the house up; then it will be right as rain.
McGrawhill's American Idioms And Phrasal Verbs