Insanity

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late 16th century: from Latin insanitas, from insanus (see insane).

wiktionary

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A three-part word (root ' sane', prefix ' in-' meaning 'not', suffix ' -ity', meaning 'the state of'). Derived from Latin precursory equivalents. Two possible candidates for construction order:

etymonline

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insanity (n.)

1580s, "state of being insane, seriously impaired state of mental functioning," from Latin insanitatem (nominative insanitas) "unhealthfulness, unsoundness, disease," noun of quality from insanus "mad, insane; outrageous, excessive" (see insane). Meaning "extreme folly" is from 1844. The Latin abstract noun was insania ""unsoundness of mind, madness, frenzy."