An idiom that means, ostensibly, an untruth with no hair on its face, and refers to a particularly egregious lie. As amusing as this mental image is, it's incorrect. Well, I shouldn't say that--after all, usage determines meaning--but it's not the original intent of the phrase. The original phrase was "bold-faced lie." Before you picture a lie with fluorescent clown makeup (oops, too late!), you should know that this refers to printing. As in, boldface type. But a bold-faced lie is not just "any lie which has been printed in bold face," e.g.
The Backstreet Boys Don't Suck and They Never Have!
No, I'm afraid it actually just means, "That's a
lie," with the emphasis on the word to denote the seriousness of the
transgression.
The reason it's misspoken as "bald-faced lie" is because it almost never appears written out in print. Just like the phrases "with a capital," or "period," it refers to a way of printing the word, and therefore is almost never seen written out.