Yours, Etc.: Origins and Uses of 8 Sign-Offs
1670s, "showy brilliance," from French éclat "splinter, fragment" (12c.), also "flash of brilliance," from eclater "burst out, splinter," from Old French esclater "smash, shatter into pieces," of uncertain origin, perhaps from a West Germanic word related to slit or to Old High German sleizen "tear to pieces; to split, cleave." Extended sense of "conspicuous success" is first recorded in English in 1741.