The Spanish word 'alcaldía' (meaning 'mayor's office' or 'city hall') has an interesting journey through Arabic into Spanish. It starts with the Classical Arabic word 'qaḍi' meaning 'judge', which became 'alqáḍi' in Andalusi Arabic (the Arabic spoken in medieval Islamic Spain). This term evolved into the Spanish word 'alcalde' meaning 'mayor'. The final form 'alcaldía' was created by adding the Spanish suffix '-ía', which is used to form nouns indicating an office or jurisdiction, similar to how English uses '-ship' or '-dom' (like in 'mayorship' or 'kingdom').

This etymology reflects the historical influence of Arabic on Spanish during the centuries of Islamic presence in the Iberian Peninsula. The semantic shift from 'judge' to 'mayor' makes sense historically, as both roles represented local authority figures in their respective cultures.

The most directly related Spanish word is 'alcalde' meaning 'mayor', which is actually the base word from which 'alcaldía' is derived. You might also encounter this word more frequently as it refers to the person holding the office, while 'alcaldía' refers to the institution or building itself.

While English doesn't have any direct cognates from the Arabic 'qaḍi', we do have some borrowed words that entered English through Arabic and Spanish contact, such as 'alcalde' itself, though this is rarely used in English except when discussing Spanish-speaking contexts. The suffix pattern is similar to English words ending in '-cy' that denote office or jurisdiction, like 'presidency' or 'magistracy', though these come from Latin rather than Arabic origins.