The word eggcorn was coined by the Language Log linguists and indicates odd spellings of words (like eggcorn for acorn), which mean that they can be interpreted differently. For a comprehensive history of the term, please click here.
According to Geoff Pullum, eggcorns could be dismissed "as signs of illiteracy and stupidity, but they are nothing of the sort. They are imaginative attempts at relating something heard to lexical material already known. One could say that people should look things up in dictionaries, but what should they look up? If you look up eggcorn you’ll find it isn’t there. Now what? And you can’t look up everything; sometimes you think you know what you just heard and you don’t need to look it up."
Well, that makes me feel a whole lot better about my "dull as a dishwasher". Chris Waigl has indicated in the comments on this site that she is investigating this expression, so I might have the pleasure to contribute to her eggcorn database. She has given the delightful name of poteaux roses (pot aux roses, from the expression découvrir le pot aux roses meaning "to find out what’s going on") to the French eggcorns she’s hoping to chase as well. I also highly recommend Chris’s blog Serendipity.