(come) rain or shine = whatever the weather; no matter what happens
e.g. my brother runs 10km everyday come rain or shine; nothing can prevent him.
The earliest recorded use of the idiom was in “Astro-meteorologica, or Aphorisms And Large Significant Discourses on the Natures and Influences of Celestial Bodies by” John Goad (1699).
The idiom has been used since to express the certainty that something will happen irrespective of the weather, circumstance, condition or situation.
Modern usage: “Pay as little attention to discouragement as possible. Plough ahead as a steamer does, rough or smooth - rain or shine” - Maltbie Davenport Babcock (U.S. Clergyman and writer).
