Hi, My dictionary defines 'in the thick of' as 'at the busiest or most intense part of'. eg in the thick of the crowd eg in the thick of the fighting As you can see, the phrase already includes the idea of density, so adding the word 'density' is a bit like talking about 'in the dense part of the density'. This sounds rather foolish.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.