Superlative form of tough: most tough. (adjective)
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Use "toughest" in a sentence
"The broadcaster made a pre-tax profit of £300,000 last year in what it described as the toughest advertising market in its 28-year history, as it published its 2009 annual report today."
"No matter how you add it all up, Hillary Clinton says she's not dropping out of the race for what she calls the toughest job in the world."
"Obama Says Facts Support Accusation of Iranian Plot President Obama on Thursday vowed to push for what he called the "toughest sanctions" to punish Iranian officials whom he accused of complicity in a suspected plot to kill the Saudi ambassador to the United States."