verb

• (intransitive) To pronounce the consonant ‘s’ imperfectly; to give ‘s’ and ‘z’ the sounds of ‘th’ (/θ/, /ð/). This is a speech impediment common among children.
  "Until the age of 10, Dominic would lisp, but this was fixed by a speech therapist."
• (intransitive) To speak with imperfect articulation; to mispronounce, such as a child learning to talk.
  "As yet a Child, nor yet a Fool to Fame, / I liſp'd in Numbers, for the Numbers came."
• (intransitive, archaic) To speak hesitatingly and with a low voice, as if afraid.
  "Lest when my lisping, guilty tongue should halt."
• (intransitive, archaic) to express by the use of simple, childlike language.
  "But the fashion spreads deeper and wider; the village is infected and the village green; Amelias and Claras sweep your rooms and cook your dinners, gentle Sophias milk your cows, and if you ask a pretty smiling girl at a cottage door to tell you her name, the rosy lips lisp out Caroline."