This is the 7111th most frequent Chinese word.
硬骨头
"硬骨头" means "a tough person who refuses to yield or compromise."
The term '硬骨头' is used metaphorically to describe someone with a resolute and unyielding character.
他是个硬骨头，从不向困难低头。
He is a tough person and never bows to difficulties.
Here, '硬骨头' is used to literally describe something that is hard, in this case referring to the texture of the pickle.
这块泡菜不够腌透，吃起来像硬骨头。
This piece of pickle is not well permeated; it feels like a hard bone when eating.
In this sentence, '硬骨头' is used symbolically to inspire firmness and perseverance in difficult situations.