1
general::
Also, get or have the jump on. Achieve a distinct advantage over someone, especially through early or quick action; get a head start. For example, Their book on electronic communication has the drop on all the others, or We really got the jump on the competition. The first of these slangy expressions dates from the mid-1800s and originally alluded to pointing one's gun at someone before he pointed his at you. It was transferred to more general use by about 1900. The second, which uses jump in the sense of "start," dates from about 1900.
American Heritage Idioms
2
general::
Phrase(s): get the drop on someone
1. Sl. [for person A] to manage to get a gun aimed at person B before person B can aim back at person A. (The gun is then "dropped" by person B.) • Fred got the drop on Wilbur in a flash. • Wilbur was too stoned to get the drop on Fred. 2. Sl. to succeed in getting an advantage over someone. • I guess I got the drop on you because I was early. • I got the drop on almost everybody by sending in my registration by e-mail.
McGrawhill's American Idioms And Phrasal Verbs