tough nut to crack, A difficult problem or person to deal with.
nuts and bolts, The basic, practical details of something.
Word Origin
Language
Old English
Original word
hnutu
Originally meant
hard fruit
Background
The word 'nut' comes from the Old English 'hnutu', which referred to the hard-shelled fruit of plants. Over time, it has also come to describe the metal fastener and a person with an intense enthusiasm for something.