Why is this question not GR?
What phrase is “o'clock” contracting?
Of or according to the clock: three o'clock.
c.1720, abbreviation of of the clock (1640s), from M.E. of the clokke (late 14c.). Attested from 1904 in reference to direction (by shooters, fighter pilots, etc.).
How was this relevant comment ignored?
Flagged as general reference because a Google search for "o'clock" leads directly to several pages explaining it including three dictionaries. – Hugo
How could the ever-vigilant members let the question garner twelve up votes, and the answers 18+7+5+1 up votes?
Above all, what have the answers contributed beyond what is already available as general reference?