We don't have synonyms for hardihead yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are hardihood, hardhead and hard head.
(n) :
(countable) Alternative form of hardyhead (“the fish”). [(Australia) Any of the sundry species of fish of the family Atherinidae found near Australia;]
(n) :
(obsolete) Alternative form of hardihood. [Unyielding boldness and daring; firmness in doing something that exposes one to difficulty, danger, or calamity; intrepidness.]
(n) :
(countable) Alternative form of hardyhead (“the fish”). [(Australia) Any of the sundry species of fish of the family Atherinidae found near Australia;]
(n) :
(obsolete) Alternative form of hardihood. [Unyielding boldness and daring; firmness in doing something that exposes one to difficulty, danger, or calamity; intrepidness.]