aka THE WARRIOR MONK [never married ] "Mad Dog, as in ‘Mad Dog’ Mattis: The colorful history of a great American nickname " The Washington Post

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The 66- year -old general , called a " warrior monk " by his peers for his depth of knowledge and lack of family — he never married — is also known to turn a memorable phrase , including : " Be polite , be professional , but have a plan to kill everybody you meet . "
And if the nickname Mad Dog gives you pause , well , the retired general does not like it much either,..."

https://www.washingtonpost.com/news/morning-mix/wp/2016/12/02/trumps-pick-for-secretary-of-defense-is-known-as-mad-dog-heres-the-history-of-the-nickname/?utm_term=.cc39cac4154c