Why is the word for lisping one that lispers can't pronounce?

I always wondered this. It's true in more than one language, as well...in Russian, the inability to properly roll the r in a word (an essential part of speaking Russian) is called...argh, I think it's kartavyet with a rolled 'r'- according to jrn and 'view source', that's written "картавить." To add insult to injury, the use of the common workaround (which sounds like a hard ch) is called grassiriyu, again with rolled r's.

I don't speak Russian, ergo I've probably reversed the two terms, and definitely massacred their phonetic spellings. My apologies to the Russian language.

The reason for this is that these words are onomatopoeic--a term that almost nobody can pronounce. So you see, it all turns out fair at the end.

It's a germanic word - dating at least from the Old English days when one declension of the word was wlyspian. German is lispen.

It almost certainly was never said or needed to be said by the afflicted ones themselves (same as for stutterers/stammerers), who might rarely have said anything for fear of retribution. They were probably looked upon as being soft-minded and fit only for low jobs, or sacrifices to the gods.