formido

The verb is probably derived from the noun.^[1] Their origin is unknown:

• Possibly from the root Proto-Indo-European *dʰer- (“to hold”) (the same of firmus), as the fear that makes one rigid.
• Alternatively cognate with Ancient Greek μόρμορος (mórmoros, “fear, panic”), μύρμος (múrmos, “fear”) and μορμώ (mormṓ, “bogey, she-monster”), with the same dissimilation *morm- > form- seen in formīca and possibly fōrma.^[2][1]
  If this is of Proto-Indo-European origin, it would appear to be a reduplication of a root *mer-, possibly the same root as *mer- (“to disturb”) and/or the source of *mor- (“incubus, mare (evil female spirit of nightmares)”) (which is otherwise thought to be from *mer- (“to disappear, die”)). (Can this⁽⁺⁾ etymology be sourced?) However, de Vaan reconstructs only a stem *mormo- which he does not allow an etymology.
  If not Indo-European, then a substrate origin must be considered, which the variation of forms in Greek and morphological obscurity in both languages make likely. (Can this⁽⁺⁾ etymology be sourced?)

The suffix component -īd- is unexplained in either case, but compare -idus (with different vowel length), -ō (forming nouns).

• (Classical Latin) IPA^(key): /forˈmiː.doː/, [fɔrˈmiːd̪oː]
• (modern Italianate Ecclesiastical) IPA^(key): /forˈmi.do/, [forˈmiːd̪o]

formīdō (present infinitive formīdāre, perfect active formīdāvī, supine formīdātum); first conjugation

1. to dread or fear
2. to be afraid of or for

• afformīdō
• performīdō
• praeformīdō
• reformīdō

formīdō f (genitive formīdinis); third declension

1. fear, alarm, terror, dread
2. fright, horror
3. scarecrow

Third-declension noun.

• formīdābilis
• formīdāmen
• formīdulōsus

• “formido”, in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
• “formido”, in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
• formido in Gaffiot, Félix (1934) Dictionnaire illustré latin-français, Hachette.
• Carl Meißner, Henry William Auden (1894) Latin Phrase-Book‎^[1], London: Macmillan and Co.
  • superstitious fears; phantoms: formidines
    
• Pokorny, Julius (1959) Indogermanisches etymologisches Wörterbuch [Indo-European Etymological Dictionary] (in German), volume 2, Bern, München: Francke Verlag, page 749