'unbeknownst' originates from Middle English, specifically the word 'beknown,' where 'be-' meant 'by' and 'known' meant 'to know.'

'beknown' changed from Middle English word 'beknowen' and eventually became the modern English word 'unbeknownst.'

Initially, it meant 'known by someone,' but over time it evolved into its current meaning of 'without the knowledge of someone.'