Etymology of 若 vis-a-vis 如果 I came across 若 from my lessons today, as in


  若天下雨，我就出发。


Being familiar with 如果, I immediately assumed 若 is just a short/abbreviated/spoken form of 如果.

However, I broached the idea to my Chinese colleagues and they immediately pointed out instead that 若 in practice is a bit more stodgy version of 如果 primarily used among "elite" or pretentious types (more "formal" was their wording).

Nevertheless, this left me wondering if perhaps there's some historical truth to my initial suspicion.
It's not likely that 若 is a contraction of 如果, because 若 came first by a thousand years or so.

Nor is it likely that 如果 is a splitting of 若, because 果 is real. Long before 如果 there was already 若果, and before 若果, 果 was already grammaticalized to mean "ultimately" ("bear fruit" -> "ultimately", see 果然, 果真).

So 若果 was a legitimate, intensified version of 若. As for how 若果 became 如果, we can speculate that it was an easy phonetic and etymological transition: 如果 [nja kojʔ] and 若果 [njak kojʔ] would have sounded almost identical even before the loss of stop finals post-MC, and 如 ("follow" -> "compares with/likens to") and 若 ("obey" -> "as if/if") are grammaticalized in similar, if subtly different ways.