1591, "bright, shining," from L.
lucidus "light, bright, clear," from
lucere "to shine," from
lux (gen.
lucis) "light," from PIE base
*leuk- "to shine, be bright" (see
light (n.)). Sense of "easy to understand" first recorded 1786.
Lucid interval "period of calm or temporary sanity" (1581) is from M.L.
lucida intervalla (pl.), which was common in medieval Eng. legal documents (cf.
non est compos mentis, sed gaudet lucidis intervallis).