'legally-gained' originates from the combination of 'legal' and 'gain', where 'legal' is derived from Latin 'legalis', meaning 'pertaining to the law', and 'gain' from Old French 'gaigner', meaning 'to earn or win'.

'legalis' transformed into the Old French 'legal', and 'gaigner' evolved into the Middle English 'gainen', eventually forming the modern English 'legally-gained'.

Initially, 'legal' meant 'pertaining to the law', and 'gain' meant 'to earn or win'. The combination 'legally-gained' has maintained its meaning of 'obtained in accordance with the law'.