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1.
1) A greasy substance thicker than Vaseline that you rub on your horse.

2) A exclamatory statement in response to something outlandish or unbelievable.

3) The image of grease being rubbed on a manure covered horse.
Holy horse rub!
That's horse rub, I don't believe you!
You look like you've been horse rubbed!
by Birdie B January 03, 2013