Mondegreens are misheard versions of phrases, sayings, lyrics, poetic phrases, or slogans. The term comes from the Scottish author Sylvia Write, who wrote a 1954 article in Harper’s Magazine in which she mentioned misinterpreting a Scottish ballad. The original line, “They hae slain the Earl o’ Moray and laid him on the green,” she misheard as, “They hae slain the Earl o’ Moray and Lady Mondegreen.”
Although the term mondegreen has been used for misheard phrases not from songs and poems, eggcorn, which originated in a 2003 Language Log post, has been advanced as a broader term for misheard words or phrases that retain their original meanings. So, for example, doggy-dog world is an eggcorn because it’s used in roughly the same way as the original phrase, dog-eat-dog world.