Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
drop 2 (n.)
combines to make several important idioms: a drop in the bucket is a cliché for “the infinitesimal,” “the trivial.” At the drop of a hat means “at the slightest provocation,” as in He’ll fight at the drop of a hat, also a cliché. To get the drop on is slang, originally gunslingers’ or duelists’ jargon meaning literally “to get one’s gun aimed at the enemy before the enemy can take aim at you” and figuratively “to get an advantage over someone,” as in I got the drop on him by turning in my request first. The expression probably stems from the duelists’ practice of awaiting the drop of a handkerchief by a third party as the signal to aim and fire; to get the drop was to be the first to respond to the signal.
