The quality or state of being fanatical; excessive or irrational devotion to a cause, belief, or activity.
From 'fanatical' plus the -ness suffix, which forms abstract nouns from adjectives. The root traces back through 'fanatic' to Latin 'fanaticus,' originally meaning temple-possessed or divinely frenzied.
The shift from 'fanaticalness' being a spiritual virtue (divine inspiration) to a moral flaw (dangerous obsession) happened gradually as societies became more secular—the same behavior got rebranded as civilization progressed.
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