A difficult problem or situation that is hard to solve

एक कठिन समस्या या परिस्थिति जिसका समाधान कठिन हो

English Usage: Getting him to agree to this plan is going to be a tough nut to crack.

Hindi Usage: उसे इस योजना पर सहमत कराना एक कठिन समस्या साबित होगी।