The Collaborative International Dictionary of English v.0.48:
Lisp \Lisp\ (l[i^]sp), v. i. [imp. & p. p. Lisped (l[i^]spt);
p. pr. & vb. n. Lisping.] [OE. lispen, lipsen, AS. wlisp
stammering, lisping; akin to D. & OHG. lispen to lisp, G.
lispeln, Sw. l[aum]spa, Dan. lespe.]
1. To pronounce the sibilant letter s imperfectly; to give s
and z the sound of th; -- a defect common among children.
2. To speak with imperfect articulation; to mispronounce, as
a child learning to talk.
As yet a child, nor yet a fool to fame,
I lisped in numbers, for the numbers came. --Pope.
3. To speak hesitatingly with a low voice, as if afraid.
Lest when my lisping, guilty tongue should halt.