What does the o in o'clock really mean?

The Real Meaning Behind O'Clock

Whenever we check the time and say it is three o'clock or five o'clock, we are actually using a piece of historical shorthand without even realizing it.

That little apostrophe hidden in the word is not there by accident. It is a traditional contraction, much like using "don't" instead of "do not" or "it's" for "it is." Back in the days when mechanical clocks were becoming common and people needed a way to distinguish between exact hour markers and other measurements, they started using the phrase "of the clock."

Over hundreds of years of everyday speech, people naturally sped up how they talked. The phrase was compressed to save breath and time, dropping letters and squishing the words together. The apostrophe was left behind to mark the exact spot where the missing words used to live, preserving a tiny footprint of history every time someone asks for the time.