a speech defect consisting in pronouncing s and z like or nearly like the th-sounds of thin and this, respectively.
2.
Phonetics. any unconventional articulation of the sibilants, as the pronunciation of s and z with the tongue between the teeth (lingual protrusion lisp), close to or touching the upper front teeth (dental lisp), or raised so that the breath is emitted laterally (lateral lisp).
late O.E. awlyspian, from wlisp (adj.) "lisping," probably of imitative origin (cf. M.Du., O.H.G. lispen, Dan. læspe, Swed. läspa). The noun is first attested 1625.
lisp (lĭsp) n. A speech defect or mannerism characterized by mispronunciation of the sounds (s) and (z) as (th) and (th). v.lisped, lisp·ing, lisps To speak with a lisp.