A disastrous event has extremely bad consequences and effects. the recent, disastrous earthquake = catastrophic + disastrously dis·as·trous·ly The vegetable harvest is disastrously behind schedule
(of events) having extremely unfortunate or dire consequences; 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"
Attended with suffering or disaster; very unfortunate; calamitous; ill-fated; as, a disastrous day; a disastrous termination of an undertaking
If you describe something as disastrous, you mean that it was very unsuccessful. their disastrous performance in the general election of 1906. + disastrously dis·as·trous·ly the company's disastrously timed venture into property development. very bad, or ending in failure