Etymology
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mirabile dictu (interj.)

Latin, literally "wonderful to relate," from neuter of mirabilis "wonderful, marvelous, extraordinary; strange, singular" (see marvel (n.)) + ablative supine of dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly"). The expression is found in Virgil. Mirable "wonderful, marvelous" was used in English 15c.

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horribile dictu 
Latin, "horrible to say, dreadful to relate," from neuter of horribilis (see horrible) + ablative supine of dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly").
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