This phrase comes from someone who always has or looks like they have a five o'clock shadow on their double chin.

Used to slander someone without actually letting them know why or what about. A nickname someone might think is cool but is really disgraceful.
Bill: Hey frank, Charlie always looks like he has a five o'clock shadow on his double chin.

Frank: Lets call him Five O'Clock Dub, bet he will think its awesome!
by Dublicious January 15, 2012
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