'in-law' originates from Old English, specifically the word 'inlagu,' where 'in-' meant 'within' and 'lagu' meant 'law.'

'inlagu' transformed into the Middle English word 'inlawe,' and eventually became the modern English word 'in-law.'

Initially, it meant 'within the law,' but over time it evolved into its current meaning of 'relatives by marriage.'