You know, I did know that it means “The devil you say!” and I knew it connoted disbelief, but I never really understood why it means that. It’s a bit strange. (I know a lot more than I let on, in most cases.)

http://www.proz.com/kudoz/english/poetry_literature/1132287-the_devil_you_say.html#2713657

Response from Russ Cable (Dallas, TX – U.S.A.)
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Greg, The word ‘deuce’ is a euphemism for the devil and dates back to the late 17th century (the similarly used ‘dickens’ dates from the late 16th century) and has been used in oaths, especially, ‘what (who, why, etc.) the duece,’ usually expressing impatience or irritation, and in ‘ the deuce to pay.’ ‘Deuce’ derives from Low German ‘duus,’ and probably from the sense ‘a throw of two at dice (the lowest-scoring throw with two dice), and hence denoting the embodiment of bad luck. It was used as a mild substitution for devil back when it was considered profane to speak the name.

I’m not sure when ‘the deuce you say’ first appeared, but the following is an example used by Dumas in his ‘Count of Monte Cristo’ (1844), Chapter 9: “Then give me a letter to him, and tell him to sell out without an instant’s delay, perhaps even now I shall arrive too late.” “The deuce you say!” replied the marquis, “let us lose no time, then!

From what I can gather from the Google contexts in which it appears, ‘the deuce (devil) you say’ means ‘really’ or ‘you’re kidding me (or no kidding),’ or ‘is that a fact?’

A refrigerator magnet distributed at the 2004 “Sound Of The Baskervilles’ Master’s Dinner” read: “Watson, I’m afraid I’m 150 this year.” “The deuce, you say, Holmes! Why you scarcely look 100!!”

(Oxford Dictionary of Slang) ”

As far as a countdown goes, you’ll have to count backwards.