...he poured himself a jigger of whiskey and swallowed it neat...
-- Truman Capote, Breakfast at Tiffany's, 1958
"Now, mates," I cried, "let's get upon the fore-top-sail yard and see what we can do there." And up we went, and in three quarters of an hour, with the help of a jigger, we had hauled out the earrings and tied every blessed reef-point in the sail.
-- William Clark Russell, The Wreck of Grosvenor, 1877
The origin of jiggers is unknown, though it likley entered English in the late 1600s.