From an unused root (apparently meaning to act harshly); violent; by implication deadly; also (in a good sense) brave -- cruel, fierce.
H393. akzar
אַכְזָר adjective cruel, fierce, of poison (ראֹשׁ) Deuteronomy 32:33; elsewhere of men, fierce Job 41:2; as substantive = a cruel one 30:21 (of God), Lamentations 4:3.