Index
late Middle English: from Latin index, indic- ‘forefinger, informer, sign’, from in- ‘towards’ + a second element related to dicere ‘say’ or dicare ‘make known’; compare with indicate. The original sense ‘index finger’ (with which one points), came to mean ‘pointer’ (late 16th century), and figuratively something that serves to point to a fact or conclusion; hence a list of topics in a book (‘pointing’ to their location).
wiktionary
From Latin index(“a discoverer, informer, spy; of things, an indicator, the forefinger, a title, superscription”), from indicō(“point out, show”); see indicate.
etymonline
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" (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.
index (v.)
"compile an index," 1720, from index (n.). Related: Indexed; indexing.
