disastrous
adj.adj. causing a lot of damage, failure, or pain. You use this to describe something that goes very wrong.
adj. causing great damage, loss, or misfortune; highly unsuccessful or catastrophic in outcome.
The heavy rain had a disastrous effect on the crops.
The company's decision to ignore customer feedback proved disastrous for their reputation and sales.
Historians often debate whether the general's disastrous retreat was a result of poor intelligence or a fundamental misunderstanding of the local terrain.
From disaster + -ous, after Middle French desastreux (modern French désastreux; from desastre (modern French désastre, “disaster”; from des- + astre, a calque of Italian disastro) + -eux), itself after Italian disastroso, from disastro + -oso, from dis- (“away, without”) (from Latin dis-) + astro (“star, planet”) (from Latin astrum (“star, celestial body”), from Ancient Greek ᾰ̓́στρον (ắstron)).
Commonly modifies nouns like 'consequences', 'results', 'effects', or 'failure'.