Part of speech: Adjective (satellite)
(of events) having extremely unfortunate or dire consequences
Example: bringing ruin; the stock market crashed on Black Friday; a calamitous defeat; the battle was a disastrous end to a disastrous campaign; such doctrines, if true, would be absolutely fatal to my theory"- Charles Darwin; it is fatal to enter any war without the will to win it"- Douglas MacArthur; a fateful error
The word "calamitous" comes from French calamiteux.
For 300 years after 1066, French was the language of English courts and nobility.