Used to say that something will happen or be true only if another thing does not happen or is not true. It introduces an exception to a statement.
From 'on les' or 'on lesse' in Middle English, meaning 'on less (condition that)'. Over time the phrase fused into 'unless'.
Unless is basically 'if not' in a single word: 'I’ll go unless it rains' = 'I’ll go if it doesn’t rain'. Once you see it that way, many confusing sentences with 'unless' suddenly become clearer.
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