The phrase "tough exterior" means someone appears strong and unyielding on the outside, even if they may have underlying vulnerabilities or emotions. Full definition
I tried so many things to get them to cook through but everything resulted in an extremely tough exterior and a mushy inside. (deliciouslyella.com)
The bell pepper's tough exterior skin may be difficult for some dogs to chew and digest. (topdogtips.com)
Similar to fleas, ear mites lay eggs that have an extremely tough exterior, which makes it difficult to kill ear mites in one treatment. (1800petmeds.com)