The Spanish word 'fornicario' (relating to fornication) comes from Late Latin 'fornicarius' with the same meaning. This derives from the Latin verb 'fornicari' meaning 'to visit brothels', which itself comes from the Latin noun 'fornix' meaning 'arch' or 'vault'. This interesting semantic development occurred because Roman prostitutes often waited for customers under the vaulted archways of buildings, particularly in ancient Rome. Over time, the architectural term came to be associated with prostitution and eventually with illicit sexual activity in general.

A related Spanish word is 'fornicar' (to fornicate), which is more commonly used than 'fornicario' and shares the same Latin root. The noun form 'fornicación' (fornication) is also related.

English speakers might recognize the connection to the words 'fornicate' and 'fornication', which come from the same Latin roots. The architectural term 'fornix' also survives in English as a technical term in anatomy, referring to various arch-shaped structures in the body, such as the 'fornix of the brain'.