means To pronounce the sounds 's' and 'z' as 'th', or the speech impediment that produces this softened, slushy sibilance.

from From Old English 'wlispian,' to speak with a stammer or lisp, related to a small family of Germanic words for stuttering and faltering speech (compare Old High German 'lispen'). The word is widely thought to be imitative — its own soft, slippery sound mimicking the very mispronunciation it names. The 'w' fell silent and dropped away over the centuries, leaving the gentler 'lisp' we say today.