'illegally-kept' originates from the combination of 'illegal,' which comes from Latin 'illegalis,' meaning 'not legal,' and 'kept,' the past participle of 'keep,' from Old English 'cēpan,' meaning 'to seize or hold.'

'illegal' changed from the Latin word 'illegalis' and 'kept' from Old English 'cēpan,' eventually forming the modern English term 'illegally-kept.'

Initially, 'illegal' meant 'not legal,' and 'kept' meant 'held or maintained.' The combination 'illegally-kept' evolved to specifically describe something maintained unlawfully.