'illicitly-kept' originates from the word 'illicit,' which comes from Latin, specifically the word 'illicitus,' where 'in-' meant 'not' and 'licitus' meant 'lawful.'

'illicitus' transformed into the Old French word 'illicite,' and eventually became the modern English word 'illicit' through Middle English.

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