Its source is glan da, a Welsh name meaning "Sanctified good."
This name is a modern creation, popularized in the 20th century by the British actresses Glenda Ferrell and Glenda Jackson.
How many times have you seen the 1939 film The Wizard of Oz? Enough times to remember the character of Glenda the Good Witch?
There is no such character in the film. The powder-blue fairy, played by Billie Burke, was named Glinda, possibly a blend of Glenda and Linda. Given Glenda's meaning (sanctified good) and Linda's (pretty), it is difficult to imagine a more agreeable blending of names for a Good Witch.