see hard nut to crack.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use tough nut in a sentence
Altogether, it is a very tough nut to crack, and the force at our disposal none too strong for the work that lies before it.The Invasion | William Le Queux
Drank rum by the hogshead, pounded his wife till she left him, and was a tough nut gin'rally.Cap'n Eri | Joseph Crosby Lincoln
"H'm, and a pretty tough nut to crack," threw in Cleek with a little laugh.The Riddle of the Purple Emperor | Mary E. Hanshew and Thomas W. Hanshew
Point is that Houck, who always was a t-tough nut, has gone bad since the boys rode him on a rail.The Fighting Edge | William MacLeod Raine
This particular member of the force was an unusually tough nut to crack.Oh, You Tex! | William Macleod Raine