Coined by Dr Evil in the movie Austin Powers in Goldmember, tough sub is an alternate for "tough crowd", for which the meaning is exactly the same.
Dr Evil: "Ladies and Gentlemen welcome to my submarine lair. It's long and hard and full of sea men!"

*silence*

Dr Evil: "No? Nothing? Not even a titter? Tough sub."
by orders normal lucy May 17, 2007
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