the drop

http://wonderfulstoriesonline.blogspot.hk/2009/01/adventure-of-dancing-men-11.html

It was all done so swiftly and deftly that the fellow was helpless before he knew that he was attacked. He glared from one to the other of us with a pair of blazing black eyes. Then he burst into a bitter laugh.

"Well, gentlemen, you have the drop on me this time. I seem to have knocked up against something hard. But I came here in answer to a letter from Mrs. Hilton Cubitt.
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Does it mean this time you have attacked me? DROP in this sense is not found in dictionaries; I just guess ATTACK on the strength of ON.

Does the second red part mean I come across some obstacle?