An archaic spelling variant of ecstasy; a state of overwhelming joy, wonder, or trance-like condition.
From Greek ekstasis (standing outside oneself), from ek- (out) + stasis (standing, state). This is an old English spelling that preserves the Greek etymology more directly than modern 'ecstasy.'
Medieval poets spelled it this way to make the Greek roots more obvious—'extacie' literally shows you 'out of stasis,' which is exactly what ecstasy is: your mind leaving its normal state.
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