Problem with idiomatic expression

[deviputinoso]

Hello everybody!

I am very happy I had found this forum. I am an Spanish speaking person but I love to read English books in the original. Reading one of these, I found the following espression I could not translate by any means: "I have got the drop on you, gumshoe".

The only part I could find in the dictionary (English dictionary was gumshoe - detective. I will apreciate if someone can help me, because I am very intriged by this expression.

[Tdol]

If you get the drop on someone, you strike first.

[deviputinoso]

Thanks, tdol!

[ebb]

The bad guy's pulled out his piece before Sam Spade (or Mike Hammer) can do so... the villain was quicker on the draw. From the context, this mug's apparently snuck up behind Sam (or Mike) and could have bushwhacked him, but he couldn't resist rubbing Sam's nose in it first.

And, strictly on the QT, even though this con's got a lot of moxie, it's always a boner to mess with our hero: This joker will be munching on a knuckle sandwich soon, or eating hot lead from Mike's .45.

Now, unwind all of that, and you're ready for your PI license.

("Snuck" = past tense of "sneak" in American slang, but don't try this one with your English professor.)