jigger (n.) Look up jigger at Dictionary.com
"1.5-ounce shot glass," 1836, American English, in early use also of the drink itself, from jigger "illicit distillery" (1824), of unknown origin; or else perhaps from jigger, a 1756 alteration of chigger "tiny mite or flea." As a name for various appliances, the word is attested by 1825, from jig.