From pistis (faith/trust). Genuine, pure, trustworthy — describing the quality of the expensive nard (spikenard) ointment poured on Jesus.
Mary anointed Jesus with 'pure nard, an expensive perfume' (Mark 14:3, John 12:3). The adjective emphasizes the ointment was genuine, not counterfeit — the real thing, not a cheap imitation. This mirrors the gospel call: genuine faith, genuine love, genuine worship. God is not honored by imitations.