The Spanish word 'diario' comes from the Latin word 'diarium' meaning 'daily journal'. This Latin word was formed by combining two parts: 'dies' meaning 'day' and the suffix '-arium' meaning 'belonging to'. So literally, it meant something 'belonging to the day' or 'daily'. This combination perfectly reflects how a diary is something we write in day by day, and how the adjective 'diario' describes things that happen daily.

Several common Spanish words share the same Latin root 'dies' (day): 'día' (day), 'mediodía' (noon/midday), and 'cotidiano' (daily/everyday). You can see how all these words relate to the concept of 'day', just like 'diario'.

The English word 'diary' is a direct cousin of Spanish 'diario', as both come from Latin 'diarium'. English also has several words containing the same day-related root, such as 'diurnal' (active during the day), 'meridian' (midday), and 'journal' (which originally meant a daily record, from French 'journal', ultimately from the same Latin root).