Index is Latin kept whole, endings and all, which is why its plural can still be indices. The Latin word means "one who points out" — a discloser, an informer, a pointer, a sign — from indicare, "to point out," from in- plus dicare, "to proclaim," the root behind diction and dictate.
Its first English meaning, in the late 1300s, was the forefinger, because that is the finger you point with. That sense is now obsolete except in the compound index finger, recorded from 1768. The meaning everyone knows, an alphabetical list of a book's contents with directions where to find them, is from the late 1500s, taken from Latin book-titles like Index Nominum, "Index of Names." A pointer on an instrument or the hand of a clock is from the 1590s, and the scientific senses, like refractive index, from the 1700s.
index (n.) 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" (see in- (2)) + 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. also from late 14c. index (v.) "compile an index," 1720, from index (n.). Related: Indexed ; indexing . also from 1720