c. 1200, "bold, daring, fearless," also "presumptuous, audacious," from Old French hardi "bold, brave, courageous; confident, presumptuous," from past participle of hardir "to harden, be or make bold," from Frankish *hardjan "to make hard" (source also of Old Frisian herda, Old High German herten, Old Norse herða, Gothic gahardjan "make hard"), from Proto-Germanic *hardu- (from PIE root *kar- "hard"). Sense influenced by English hard. Of plants, "able to survive in the open year-round," 1660s. Related: Hardily; hardiness. Hardhede "physical hardiness" is attested from early 15c.
word-forming element meaning "state or condition of being," from Old English -had "condition, quality, position" (as in cildhad "childhood," preosthad "priesthood," werhad "manhood"), cognate with German -heit/-keit, Dutch -heid, Old Frisian and Old Saxon -hed, all from Proto-Germanic *haidus "manner, quality," literally "bright appearance," from PIE (s)kai- (1) "bright, shining" (Cognates: Sanskrit ketu "brightness, appearance"). Originally a free-standing word (see hade); in Modern English it survives only in this suffix.
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Definitions of hardihood from WordNet
the trait of being willing to undertake things that involve risk or danger;