'o'clock' originates from the phrase 'of the clock', where 'of' meant 'belonging to' and 'clock' referred to the timekeeping device.

'of the clock' transformed into the contracted form 'o'clock' in Middle English, and eventually became the modern English word 'o'clock'.

Initially, it meant 'of the clock', but over time it evolved into its current meaning of specifying the hour.