Origin of Phrases - R
Right as rain
Meaning: Completely correct, proper, sound and healthy.
Example: On the witness stand she appeared to be right as rain, but then she is an actress by profession.
Origin: This one is likely alliterative. The "R"'s in Right and Rain sound good together. Thanks to John Gold This comes from the idea of rainwater being pure and wholesome - well it was before pollution.
This may be reference to the value and goodness of rain which is critical to crops and farming.