jigger

jigger \JIG-er\, noun:
1. a person or thing that jigs.
2. Nautical. a. the lowermost sail set on a jiggermast. b. jiggermast. c. a light tackle, as a gun tackle.
3. any of various mechanical devices, many of which have a jerky or jolting motion.
4. Informal. some contrivance, article, or part that one cannot or does not name more precisely: What is that little jigger on the pistol?
5. Ceramics. a machine for forming plates or the like in a plaster mold rotating beneath a template.
6. Mining. a jig for separating ore.
7. a jig for fishing.
8. Golf. a club with an iron head intermediate between a mashie and a midiron, now rarely used.
9. Billiards, Pool. a bridge.
10. a. a 1½-oz. (45-ml) measure used in cocktail recipes. b. a small whiskey glass holding 1½ ounces (45 ml).
...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.