"sky-colored, sky-blue," 1660s, with -an + Latin caeruleus "blue, dark blue, blue-green," perhaps from a dissimilation of caelulum, diminutive of caelum "heaven, sky," which is of uncertain origin (see celestial). The Latin word was applied by Roman authors to the sky, the Mediterranean, and occasionally to leaves or fields. The older adjective in English was ceruleous (1570s). As a noun, from 1756. The artist's cerulean blue is from 1885.