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.
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   2. To speak with imperfect articulation; to mispronounce, as
      a child learning to talk.
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            As yet a child, nor yet a fool to fame,
            I lisped in numbers, for the numbers came. --Pope.
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   3. To speak hesitatingly with a low voice, as if afraid.
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            Lest when my lisping, guilty tongue should halt.
                                                  --Drayton.
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Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "lisping":
   blurred, breathy, choked, choking, croaking, drawling, drawly,
   dysphonic, guttural, harsh, hawking, hoarse, inarticulate,
   indistinct, mispronounced, muzzy, nasal, quavering, shaking, shaky,
   snuffling, stifled, strangled, thick, throaty, tremulous, twangy,
   velar