'illegitimately-kept' originates from the combination of 'illegitimate' and 'kept', where 'illegitimate' comes from Latin 'illegitimus', meaning 'not lawful'.

'Illegitimus' transformed into the Old French 'illégitime', and eventually became the modern English word 'illegitimate'.

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