jigger (n.1)

"1.5-ounce shot glass," 1836, American English, in early use also of the drink itself, probably from jigger "illicit distillery" (1824), a word of unknown origin. Or else perhaps from jigger (n.2) "tiny mite or flea." As a name for various appliances the word is attested by 1726, from jig. In telegraphy it was a small transformer used for regulating and maintaining the difference of potential between terminals.

"tiny mite or flea," a variant spelling of chigger (q.v.).