Kaas (כַּעַס) describes a state of inner distress that blends vexation, grief, irritation, and anger. It is the emotion of one who has been wronged, frustrated, or made to suffer — whether from external provocation or internal anguish. The verb form (kaas) means to provoke to anger, to vex, or to make bitter.
Hannah is described as in 'bitterness of soul' (kaas) when she wept and prayed at Shiloh (1 Samuel 1:6, 16) — her co-wife's provocation (kaas — deliberate vexing) created her anguish. Ecclesiastes 1:18 observes that 'in much wisdom is much grief (kaas)' — wisdom that sees the world's futility produces a sorrowing heart. The verb appears frequently for Israel's 'provoking' God to anger through idolatry — the most severe form of vexation in the OT narrative. Proverbs 12:16 contrasts the fool who quickly shows his kaas with the prudent man who ignores insult.
Kaas is the emotion of the deeply wronged — and it can move toward either healthy grief (Hannah's prayer that became faith) or destructive anger (provocation of God). The wisdom books treat kaas with nuance: grief that leads to wisdom is honored; vexation that leads to folly is warned against. The NT picks up this tension: 'Be angry and do not sin; do not let the sun go down on your anger' (Ephesians 4:26) — acknowledging the validity of this emotion while guiding its expression.