adjective

very unpleasant, serious, or shocking.

there’s been a most frightful accident

horrible,gruesome,grisly,ghastly,hideous,grim,revolting,repulsive,disgusting,horrendous,grievous,dreadful,terrible,nasty,dire,unspeakable,alarming,shocking,terrifying,harrowing,appalling,daunting,unnerving,fearful,fearsome,horrid,beastly,parlous