"relating to knowledge," especially mystical or esoteric knowledge of spiritual things, 1650s, from Greek gnostikos "knowing, good at knowing, able to discern," from gnostos "known, perceived, understood," earlier gnotos, from gignoskein "learn to know, come to know, perceive; discern, distinguish; observe, form a judgment," from PIE *gi-gno-sko-, reduplicated and suffixed form of root *gno- "to know" (see know, v.).