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Phrase(s): *tough as an old boot [and] *tough as old (shoe) leather
1. [of meat] very tough. (*Also: as ~.) • This meat is tough as an old boot. • Bob couldn’t eat the steak. It was as tough as an old boot. 2. [of someone] very strong willed. (*Also: as ~.) • When Brian was lost in the mountains, his friends did not fear for him; they knew he was tough as leather. • My English teacher was as tough as an old boot. 3. [of someone] not easily moved by feelings such as pity. (*Also: as ~.) • She doesn’t care. She’s as tough as old shoe leather. • He was born tough as an old boot and has only grown more rigid.
McGrawhill's American Idioms And Phrasal Verbs