Is it ensure, insure, or assure?
late Old English awlyspian "to lisp," from wlisp (adj.) "lisping," probably of imitative origin (cf. Middle Dutch, Old High German lispen, Danish læspe, Swedish läspa). Related: Lisped; lisping.
1620s, from lisp (v.).
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.