Read the dictionary definition of calamitous. All definitions for this word.
1. (of events) having extremely unfortunate or dire consequences; bringing ruin
1. the stock market crashed on Black Friday
2. a calamitous defeat
3. the battle was a disastrous end to a disastrous campaign
4. such doctrines, if true, would be absolutely fatal to my theory
5. it is fatal to enter any war without the will to win it
6. a fateful error