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o'clock (adj.)

phrase preceded by one, two, three, etc., and signifying the time of day as shown by the face of a clock, c. 1720, an abbreviation of of the clock, from Middle English of the clokke (late 14c.). See O + clock (n.). The use of clock hand positions to describe vector directions or angles is from late 18c.

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Definitions of o'clock from WordNet

o'clock (adv.)
according to the clock;
it's three o'clock in Tokyo now
From wordnet.princeton.edu