“Nut” has two meanings: tuerca, nuez. You may know these. People eat various types of nuts, and sometimes we have to fix something using a bolt (tornillo) and a nut. The focus here is on some idioms (modismos) and expressions that use the word “nut”.
Sometimes we talk about the “nuts and bolts” of a scheme or plan (los aspectos prácticos de un proyecto). So the “nuts and bolts” of a house extension could include the specific materials and specifications.
Something can be a “hard nut to crack” (es un hueso duro de roer), meaning that it is a difficult problem. Getting Brexit right for the British government is a “hard nut to crack” given the obduracy (obstinación) of the EU and Mr. Barnier.
Somebody could be a “tough nut” (tipo duro), meaning that he is strong-minded or obstinate or both. It could be negative or positive depending on your point of view or the context.
Some mortals say that “someone is nuts” (chiflado) or crazy. Or something is “nuts”, meaning rubbish or nonsense (¡narices!). Someone might “go nuts”, that is, go crazy when, for example, they hear some news, or silly Government policy or action. Sometimes people say that someone is a “nutcase”, that he is a crazy person.
So you eat nuts, go nuts sometimes, fix something with a nut, and you have to get to grips (enfrentarse con) with the nuts and bolts of a project or plan.