google
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early 17th century: from Latin tenax, tenac- (from tenere ‘to hold’) + -ious.
wiktionary
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From Latin tenāx(“holding fast, clinging”), from tenēre(“to hold”) + -ious.
etymonline
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tenacious (adj.)
c. 1600, from Latin stem of tenacity + -ous. Related: Tenaciously; tenaciousness.
