Origin and history of witness

"one who bears testimony to faith," especially "one who willingly suffers death rather than surrender his or her religious faith," specifically "one of the Christians who in former times were put to death because they would not renounce their beliefs," late Old English martyr, from Late Latin martyr, (source also of Old French martir, Spanish martir, Italian martire, etc.), from Doric Greek martyr, earlier martys (genitive martyros), in Christian use "martyr," literally "witness."

This Greek word is sometimes said to be related to mermera "care, trouble," from mermairein "be anxious or thoughtful," from PIE *(s)mrtu- (source also of Sanskrit smarati "remember," Latin memor "mindful"); however Beekes has phonetic objections to this and suggests it is rather a loan-word from Pre-Greek.

For sense shift from abstract "testimony" to "a witness," compare French témoin "witness" from Latin testimonium; English witness (n.) "one who testifies," originally "testimony." 

The word was adopted directly into most Germanic languages (Old Saxon, Old Frisian martir, Old High German martyr, etc.), but Norse used a native formation pislarvattr, literally "torture-witness."

The meaning "one who suffers death or grievous loss in defense or on behalf of any belief or cause" (love, etc.) is from late 14c. The general sense of "constant sufferer, a victim of misfortune, calamity, disease, etc.," is from 1550s. Martyr complex "exaggerated desire for self-sacrifice" is attested by 1916.

"mental capacity, the mind as the seat of thinking and reasoning," Old English wit, witt, more commonly gewit "understanding, intellect, sense; knowledge, consciousness, conscience," from Proto-Germanic *wit-, which is reconstructed to be from PIE root *weid- "to see," metaphorically "to know" (also compare wit (v.) and wise (adj.)).

The meaning "ability to connect ideas and express them in an amusing way" is recorded by 1540s, hence "quickness of intellect in speech or writing" (for nuances of usage, see humor (n.)). The sense of "person of wit or learning" is attested from late 15c.

To be at one's wit's end "perplexed, at a loss" is from late 14c. Witjar was old slang (18c.) for "head, skull." Witling (1690s) was "a pretender to wit." Witword was "testament." Also in Middle English of the sensitive faculty generally, and sensory impressions, as wittes five for the five bodily senses (c. 1200).

Germanic cognates include Old Saxon wit, Old Norse vit, Danish vid, Swedish vett, Old Frisian wit, Old High German wizzi "knowledge, understanding, intelligence, mind," German Witz "wit, witticism, joke," Gothic unwiti "ignorance."

A witty saying proves nothing. [Voltaire, Diner du Comte de Boulainvilliers]

Wit ought to be five or six degrees above the ideas that form the intelligence of an audience. [Stendhal, "Life of Henry Brulard"]