Old English witnes "attestation of fact, event, etc., from personal knowledge;" also "one who so testifies;" originally "knowledge, wit," formed from wit (n.) + -ness. Christian use (late 14c.) is as a literal translation of Greek martys (see martyr). Witness stand is recorded from 1853.
c. 1300, "bear testimony," from witness (n.). Meaning "affix one's signature to (a document) to establish its identity" is from early 14c. Meaning "see or know by personal presence, observe" is from 1580s. Related: Witnessed; witnessing.