In a way that causes great damage, failure, or harm; with catastrophic results.
From disastrous (disaster + -ous adjective suffix) + -ly (adverb suffix). The adverb form emerged as disaster became more common in English usage.
The word 'disastrously' is often used about historical events like the Hindenburg disaster or failed space missions—it emphasizes not just that something went wrong, but that it went spectacularly, publicly wrong with major consequences.
Complete word intelligence in one call. Free tier — 50 lookups/day.