late 14c., "the forefinger," from Latin index (genitive indicis) "forefinger, pointer, sign, list," literally "anything which points out," from indicare "point out" (see indication). Meaning "list of a book's contents" is first attested 1570s, from Latin phrases such as Index Nominum "Index of Names," index expurgatorius "specification of passages to be deleted from works otherwise permitted." Scientific sense (refractive index, etc.) is from 1829; economic sense (cost-of-living index, etc.) is from 1870, from the scientific usage, from sense "an indicator." The Church sense of "forbidden books" is from index librorum prohibitorum, first published 1564 by authority of Pius IV.