We know the word candid as “truthful, honest, sincere.” It’s carried this meaning since the 1670s. But before that, it carried the meaning of “bright, white” which came from the Latin candere, which means, “to shine.”
I really kind of love this one. Because what shines? The truth. Honesty. Sincerity.
In the 1920s it came to be applied to unposed photographs–presumably, because it’s an honest glimpse of someone or something, rather than a studied, created one.