'sixties-era' originates from Modern English by combining 'sixties' (the plural of 'sixty', naming the decade) and 'era' (a word for a distinct period of time).

'sixties' comes from the numeral 'sixty' (Old English 'sixtīg' from Proto-Germanic roots meaning 'six tens'), while 'era' was borrowed into English from Latin-derived Romance languages (from Late Latin/Italian forms meaning 'age' or 'period'); the compound form 'sixties-era' arose in Modern English usage to label or describe the decade-specific period or style.

Initially it simply denoted 'the era of the sixties' (the decade itself); over time it has also come to describe styles, fashions, attitudes, or cultural elements characteristic of that decade ('sixties-era' as an attributive adjective).