Whose cruel idea was it for the word LISP, to have an "S" in it?

I know who wrote that quoye but have no idea why the "S" is in it.
My next door neighbor has a lisp and she's a speech therapist! Go figure! Except hers is a little different. I'll ask her if the pool water is warm enough for her and she'll answer : Yeth! 

LOL! Does she say Yeth often .. ?!?!

it wasnt mine