o'clock
UK[əˈklɒk]US[əˈklɑːk]
adv
Used after a number from one to twelve to indicate the hour when telling the time.
Etymology
The word 'o'clock' is a fossilized contraction of the Middle English phrase "of the clokke," meaning "of the clock." It emerged in the 15th century as mechanical clocks became more common, providing a way to specify that the time given was measured by a clock, as opposed to other methods like sundials. The apostrophe represents the lost letters 'f' and 'the' from the original phrase. Over time, the literal meaning faded, and it became the standard, unmarked way to state the hour in precise time-telling.
Analysis
The word is a contraction of the Middle English phrase "of the clock." It is not productively formed from separable modern English morphemes.
Examples
The meeting is scheduled for three o'clock.
He wakes up every morning at six o'clock sharp.
We'll meet you at the station at eleven o'clock.