From Latin 'indicare' (to point out), from PIE *deyk- (to show) — literally 'to point towards.'
To point out or show; to be a sign or symptom of; to suggest or demonstrate the need for.
From Latin 'indicātus,' past participle of 'indicāre' (to point out, to show, to make known, to declare publicly), from 'in-' (into, towards, at) + 'dicāre' (to proclaim, to dedicate, to devote), a frequentative form related to 'dīcere' (to say, to speak, to tell). The verb 'dīcere' derives from PIE *deyk- (to show, to point out), and the semantic thread is direct: to indicate is to point towards, to show by directing attention. The PIE root *deyk- produced a remarkable family of words: Latin 'dīcere' (to say — originally to show or point out with words), 'digitus' (finger — the pointer
The 'index finger' is literally the 'pointing finger' — Latin 'index' meant 'that which points out,' from the same root as 'indicate.' Before books had alphabetical indexes, they had a table at the front that 'pointed' readers toward content. The finger and the book feature share the same etymological logic: both are tools for pointing.