From pikros (bitter). Bitterness — both literal and metaphorical. The spiritual poison that corrupts the soul and defiles others.
Hebrews warns against a 'root of bitterness' growing up to defile many (Hebrews 12:15). Peter told Simon the sorcerer he was 'full of bitterness' (Acts 8:23). Paul commands believers to get rid of all bitterness (Ephesians 4:31). Bitterness is never private — it always spreads, poisoning relationships and communities.