most certain or definite; most reliable or dependable; most confident or assured.
From Old English 'sure,' from Latin 'securus' (from 'se-' without + 'cura' care). The superlative form 'surest' adds '-est' suffix indicating the highest degree.
The phrase 'the only sure things are death and taxes' comes from Benjamin Franklin—it's interesting because it connects inevitability and obligation, the two things even the richest can't escape.
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