funest
Causing death or disaster; fatal, catastrophic; deplorable, lamentable.
archaic
I do assure you, there is nothing I have a greater scorn and indignation against, than these wretched scoffers; and I look upon our neglect of severely punishing them as an high defect in our politics, and a forerunner of something very funest.
Where it comes from
From French funeste, from Latin fūnestus, from fūnus (“funeral; death”).
Adapted from the Wiktionary entry funest (CC BY-SA 4.0).