In the early days jokes were not told openly and, if there were people of the opposite sex present, they were told in whispers.

To designate these stories, at first obscene ones, the word chiste was used, derived from the verb chistar. Chistar meant “to whisper, to speak in a low voice” or also “to utter a sound with the intention of speaking”. Chistar is an onomatopoeic form that comes from the sound sst or chst, used to ask people to be quiet. The first appearance of chiste was in the 13th century used by Gonzalo de Berceo, in the form chista. From the context, we can tell that it referred to obscene stories:

Mostrad el Pater Noster a vuestras creaturas.  (Teach Our Lord’s Prayer to your children)
Castigad que lo digan yendo por las pasturas, (Punish them to say it over the fields)
Mas vale digan esso, que chistas e locuras, (It’s better for them to say it instead of jokes or nonsense)
Ca suelen tales mozos fablar muchas orruras (Because the words of such young lads are often vulgar)

Nos vemos prontito.