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 This expression is thought to have its origins in an officers' custom aboard ships sailing in the north Atlantic. In those latitudes, the sun would rise above the upper yards - the horizontal spars mounted on the masts, from which squaresails were hung - around 11 a.m. Since this coincided with the forenoon 'stand easy,' officers would take advantage of the break to go below for their first tot of spirits for the day. The expression washed ashore where the sun appears over the figurative yardarm a bit later in the day, generally after 5 p.m., and the end of the workday." From "When a Loose Cannon Flogs a Dead Horse There's the Devil to Pay: Seafaring Words in Everyday Speech" by Olivia A. Isil (International Marine/Ragged Mountain Press, McGraw-Hill, 1996)