Old English glædnes "joy; good nature;" see glad (adj.) + -ness.
Entries linking to gladness
glad adj.
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.
-ness
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).