We have an expression in dutch where to be 'on a good foot' with somebody means living in good terms with them; getting along very well. "doing the bad thing" can only imply, let's put it sweetly, the physical act of love.
It seems in your case, the original quote goes as follows: "Let's hop on the good foot and do the bad thing". After googling, I concluded it's an Austin Powers quote. Worthy of analysis? That's up to the others.
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