From Longman Dictionary of Contemporary Englishtough/hard nuttough/hard nutinformalDIFFICULT someone who is difficult to deal with He may have softened a bit in his old age but he’s still a tough nut. → nut
Examples from the Corpus
tough/hard nut• West Ham will be a tough nut to crack especially with big Lee in good form at the moment.• Already highly successful in popular music, dance and commercial television, blacks have found the movies a tougher nut to crack.• Shearer, a tough nut not inclined to whinge, said his ankle was like a pudding.• Beverley was a tougher nut to crack.• Tax will be an even tougher nut.• One glance was all it took to realise this was one hard nut to crack - his features still completely impassive.• Back, now, to the hard nuts. 