late 14c., "the forefinger," from Latin index (genitive indicis) "one who points out, discloser, discoverer, informer; forefinger (because used in pointing); pointer, sign; title, inscription, list," literally "anything which points out," from indicare "to point out," from in- "into, in, on, upon" (from PIE root *en "in") + dicare "proclaim" (from PIE root *deik- "to show," also "pronounce solemnly," and see diction). Related: Indexical.
Obsolete in English in its original sense (index finger is recorded from 1768). Meaning "alphabetical list of a book's contents with directions where in the text to find them" is from late 16c., from Latin phrases such as Index Nominum "Index of Names."
Meaning "object serving as a pointer on an instrument, hand of a clock or watch" is from 1590s. Scientific sense (refractive index, etc.) is from 1829, from notion of "an indicator." Economic sense (cost-of-living index, etc.) is from 1870, from the scientific usage.
The Church sense of "forbidden books" is from index librorum prohibitorum, first published 1564 by authority of Pius IV. The Index Expurgatorius was the catalogue of books that Catholics were forbidden to read unless certain passages were deleted, first printed 1571.
"terminal or digital member of the hand" (in a restricted sense not including the thumb), Old English finger, fingor "finger," from Proto-Germanic *fingraz (source also of Old Saxon fingar, Old Frisian finger, Old Norse fingr, Dutch vinger, German Finger, Gothic figgrs "finger"), with no cognates outside Germanic; perhaps ultimately from PIE root *penkwe- "five."
As a unit of measure for liquor and gunshot (late Old English) it represents the breadth of a finger, about three-quarters of an inch. They generally are numbered from the thumb outward, and named index finger, fool's finger, leech- or physic-finger, and ear-finger.
by 1844 as "the science of study of finger-rings," with -graphy + Latinized form of Greek daktylios "a finger ring," from daktylos "finger," which is of unknown origin. From 1884 as "finger-spelling," which earlier had been dactylology (1650s). Related: Dactylographer; dactylographic.