An eggcorn is an erroneous variant of a word, arrived at by altering one or more elements of the original word. Often the change is based on a folk etymology which means that the variant makes superficial sense to the speaker, whereas the original word was quite opaque.
A linguist, Professor Geoffrey Pullum, in an online discussion in 2003 suggested that, since there seemed to be no name for this category of words, why not use the specific example of eggcorn for acorn as a generic name.
To understand the eggcorn/acorn variation we need to go back to the Germanic words āc the oak tree and acern the acorn which are items of Old English. A folk etymology reading of acorn broke the word down into āc and corn, the latter having a basic sense of ‘a grain of sand’, and moving from that to ‘any hard seed or fruit of a plant’. So acorn is read as ‘the fruit of the oak’. If you check the picture of the acorn, you will notice the smooth rounding of the fruit, which makes eggcorn, that is, ‘a fruit that looks like an egg’, quite possible.
Eggcorns appear in languages other than English, which leads us to the wonderful word mumpsimus for the practice of persevering with a custom or practice even when the folly of doing so has been pointed out. This word comes from a Latin eggcorn, mumpsimus (which is nonsense) for sumpsimus (we have taken). The story, probably apocryphal, is recorded by Erasmus in a letter that he wrote in 1516, of a Catholic priest who, in saying the Mass, used mumpsimus for sumpsimus.
When this was pointed out to him he refused to change, either because he was too proud or stubborn to admit to his mistake, or didn’t believe the person who corrected him. This refusal to acknowledge an eggcorn is called a mumpsimus and the person who does this is also called a mumpsimus.
In the usage blog on emotional toil I mentioned a presenter who, despite hearing the phrase emotional toll, insisted on emotional toil, and could therefore possibly be a mumpsimus. It is more likely, however, that she heard what was in her own mind rather than what was in the other person’s speech.
The substitution of toil for toll is clearly an eggcorn. It is possible that the substitution of endearing for enduring (see the usage blog) is also an eggcorn although that one is not so convincing since both words have reasonable frequency, you would think.