A mondegreen site explains:

The word "mondegreen" was coined by Sylvia Wright, who wrote a column about them in the fifties, when she recounted hearing a Scottish folk song, "The Bonny Earl Of Morray." She heard the lyric, "Oh, they have slain the Earl o' Morray and laid him on the green" as "Oh, they have slain the Earl o' Morray and Lady Mondegreen."