From Latin 'candidus' (white, shining, pure) — equating honesty with luminous whiteness, cousin of 'candle.'
Truthful and straightforward; frank and honest, especially about matters that others might find unpleasant.
From Latin "candidus" (shining white, bright, radiant), from "candēre" (to shine, glow, be white-hot), from PIE *kand- (to shine, glow). This root produced Latin "candēla" (a candle, taper, whence English "candle"), "incendere" (to set fire to, whence "incendiary"), "incēnsum" (that which is burnt, whence "incense"), and Welsh "cann" (white, bright). The semantic journey from "white" to "honest" passed through Roman political culture: candidates for office wore specially whitened togas