To pronounce ”s” and ”z” sounds like ”th”. (Cambridge Dictionary)
If someone has a lisp, they pronounce the sounds ‘s’ and ‘z’ as if they were ‘th’. For example, they say ‘thing’ instead of ‘sing’. (Collins Dictionary)
If someone lisps, they say something with a lisp or speak with a lisp. (Collins Dictionary)
To pronounce the sibilants \s\ and \z\ imperfectly especially by turning them into \th\ and \t͟h\. (Merriam – Webster)
To speak falteringly, childishly, or with a lisp. (Merriam – Webster)
A speech defect in which s is pronounced like th in thick and z is pronounced like th in this. (Oxford Dictionaries)
How to cope with having a lisp.