A mondegreen (also sometimes spelled "mondagreen") is the accidental mishearing of a phrase in a poem, or song in such a way that it acquires a new, and usually humourous meaning.
In "The Death of Lady Mondegreen" from Harper's Magazine November 1954, the American writer Sylvia Wright coined it:
Ye Highlands and ye Lowlands,
Oh, where hae ye been?
They have slain the Earl of Murray,
And Lady Mondegreen.
actually, the line should have been:
And laid him on the green.