What is the origin of zeta pronunciation in spanish Spain?

The pronunciation of the letter "z" as "th" in Spain is a result of historical linguistic changes. The "z" sound in Spain is called the "ceceo", and it is believed to have originated from the influence of the Arabic language during the Muslim rule in Spain. The Arabic language has a similar sound represented by the letter "ث", which is pronounced as "th". The use of this pronunciation spread throughout Spain, particularly in southern and central regions, and it eventually became a distinct feature of the Spanish language spoken in those areas.