An "eggcorn" is a word or phrase that has been incorrectly used because the speaker or writer originally misheard the word or phrase he wanted to use and came up with something wrong that he thought was right.
"Eggcorn" is a fabricated term to describe this. A person who had heard the word "acorn" might have misheard it and then repeated the word as "eggcorn". Other terms might have been used, but "eggcorn" was the term that the linguists came up with.
Sometimes an eggcorn is a word formation that does not really exist. "Fortable", for "formidable", is an example. (Warning: "fortable" may exist somewhere with some obscure meaning, but work with me here.) Sometimes it is a real word used incorrectly: "I am internally grateful for..." is an example used in one of the cites below. There are no rules about eggcorn sightings, but it's generally accepted that the misuse has to be genuine and not the result of a typo, spellchecker substitution, or mindfart. The person uttering it has to have been under the impression that the usage was correct.
from alt. usage. english