'diagonal' originates from Latin, specifically the word 'diagonalis,' where 'dia-' meant 'through' and 'gonia' meant 'angle.'

'diagonalis' transformed into the French word 'diagonal,' and eventually became the modern English word 'diagonal' through Middle English.

Initially, it meant 'through an angle,' but over time it evolved into its current meaning of 'a line joining two opposite corners.'