1650s, "cold, icy," from French glacial or directly from Latin glacialis "icy, frozen, full of ice," from glacies "ice," probably from a suffixed form of PIE root *gel- (2) "cold, to freeze" (cognates: Latin gelu "frost;" see cold, adj.). Geological sense "pertaining to glaciers" apparently was coined in 1846 by British naturalist Edward Forbes (1815-1854). Hence figurative sense "at an extremely slow rate," as of the advance of glaciers. Related: Glacially.