I've never come across before this abbreviation but it seems everybody in business world should know it.
COP means Close of play.
In real business this term means by 5 p.m. (By the end of)
So if you have something like "... please let me have your response by COP Friday 18th". That would mean, let me have your response by 5:00pm Friday 18th.
<em>Dictionary meaning</em> (http://en.wiktionary.org/wiki/close_of_play)
close of play = (<em>idiomatic</em>) The end of the working day