Frightful means shockingly bad or alarming. A frightful mess, frightful weather, a frightful bore.
It is British and dated, and it has softened almost to comedy: a frightful mess is untidy rather than terrifying. In Old English the word meant afraid rather than frightening, which is the opposite of what it means now.
1adjectivefrightfulShockingly bad or alarming.
“The traffic in town was frightful this morning; it took me an hour to get here.”
frightful is a quality.
Old English forhtfull meant timorous or fearful. The sense flipped from feeling fear to causing it.
Old English
forhtfull — fainthearted, timorous
Middle English
frightful — afraid
English, today
frightful
From Middle English frightful (“afraid”), from Old English forhtfull (“fainthearted, timorous”). Equivalent to fright + ful.
Etymology from Wiktionary, CC BY-SA