IPA: /ˈruːɪnəsli/
KK: /ˈruːɪnəsli/
In a manner that leads to destruction or severe damage.
The company's ruinously high debts forced it to declare bankruptcy.
Ruinously → It is formed from "ruin" (from Latin "ruina", meaning a fall or collapse) and the suffix "-ously" (meaning in a manner characterized by). The word "ruinously" means in a manner that causes or results in ruin or destruction.
Think of 'ruin' as a fall or collapse, and remember that 'ruinously' describes something done in a way that leads to destruction.