Speech impediment in which a person misarticulates sibilants
A lisp is a speech impairment in which a person misarticulates sibilants ([s], [z], [ts], [dz], [ʃ], [ʒ], [t͡ʃ], [d͡ʒ]). These misarticulations often result in unclear speech in languages with phonemic sibilants. Wikipedia