1. someone/something hard to understand or deal with
the problem is a hard/tough nut to crack
Something or someone who is 'a tough nut to crack' or 'a hard nut to crack' is a problem that is challenging to solve or a person who is difficult to deal with. A person who is 'a tough nut to crack' might be difficult to understand or someone who doesn't easily reveal their true feelings to others or who can't be influenced easily. Similar in meaning to 'a tough cookie' or 'a tall order'.