The long form of this expression is 'two of the clock' or 'two
on the clock' and the apostrophe stands for the missing letters.
Two of the clock is an old expression that dates back to the
time of sundials and other means of telling time. In order to
distinguish the fact that one was referencing a clock's time,
rather than something else, one would say, "It is two of the
clock," which later became two o'clock.