The phrase "was arbitrary" implies that a decision, action, or event was made without a clear reason or justification. Antonyms for this phrase include terms like deliberate, intentional, planned, purposeful, and systematic. These words suggest that an action was taken with careful thought, intention, or strategy. Other antonyms for "was arbitrary" might include words like justifiable, reasonable, logical, or legitimate. These words suggest that an action was taken based on a clear rationale or set of criteria. By using antonyms like these, we can describe actions, decisions, and events more accurately and precisely, and convey a clearer sense of intention and purpose.