1620s, "white," from L.
candidum "white; pure; sincere, honest, upright," from
candere "to shine," from PIE base
*kand- "to glow, to shine" (see
candle). In English, metaphoric extension to "frank" first recorded 1670s (cf. Fr.
candide "open, frank, ingenuous, sincere"). Of photography, 1929. Related:
Candidly.