Say that again?

A mondegreen is the mishearing or misinterpretation of a phrase, typically a standardized phrase such as a line in a poem or a lyric in a song, due to near homophony, in a way that gives it a new meaning. 

It derives its name from Sylvia Wright's mishearing of the last stanza to the ballad "The Bonney Earl O'Moray:

Ye Highlands and ye Lowlands,

Oh, where hae ye been?

They hae slain the Earl O' Moray,

And Lady Mondegreen.

The actual fourth line is "And laid him on the green".