Old English glæd "bright, shining, gleaming; joyous; pleasant, gracious" (also as a noun, "joy, gladness"), from Proto-Germanic *gladaz (source also of Old Norse glaðr "smooth, bright, glad," Danish glad "glad, joyful," Old Saxon gladmod, in which the element means "glad," Old Frisian gled "smooth," Dutch glad "slippery," German glatt "smooth"), from PIE root *ghel- (2) "to shine." Apparently the notion is of being radiant with joy; the modern sense "feeling pleasure or satisfaction" is much weakened. Slang glad rags "one's best clothes" first recorded 1902.
