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).