'illogical' originates from Latin, specifically the word 'illogicus,' where 'in-' meant 'not' and 'logikos' meant 'pertaining to reasoning.'

'illogicus' transformed into the French word 'illogique,' and eventually became the modern English word 'illogical' through Middle English.

Initially, it meant 'not pertaining to reasoning,' and this meaning has largely remained the same in modern usage.