A lisp is a speech impediment. People with a lisp pronounce the letter 's' as 'th'. It is somewhat ironic that this handicap is called a "lisp."
In some variants of Andalusian Spanish, the lisp (ceceo) has been institutionalized as a common part of the language. For example, the word zapatos (shoes) might be pronounced as [thapatoth], contrasting with Castilian [thapatos] and American Spanish [sapatos]. According to legend, the lisp became common in Castilian because one of the Spanish kings (generally identified as Felipe V or Carlos V) spoke with a lisp, and his courtiers did not want to embarrass him by speaking otherwise. Actually, 15th-century Spanish had several phonemes that are currently rendered as s. Some dialects evolved part or all of them to th.
