"dreadful thought" is correct and usable in written English.
You can use it to describe a thought that is unpleasant or unpleasant to consider, often due to fear or anxiety. For example, "The thought of having to make a public speech filled me with a dreadful thought".
A dreadful thought occurs.
Smart had a dreadful thought: if Mitchell took her far from home and then murdered her, her parents might never know what had happened to her.
Isn't it a dreadful thought that one old Etonian could be replaced by another?" Banks's associates come from a different world.
Ostensibly, Joan's death was an accident, but Burroughs was harried by the dreadful thought that, subconsciously, he had meant to kill her.
A dreadful thought is in your mind: a bad performance a drop too many of sweat, a verbal gaffe, a sudden shiftiness in the eyes could strip away the presidential aura and mark the beginning of the end.
Perhaps it's a desire to retaliate for all the times such things have been inflicted on me by BBC Radio 4's dreadful "Thought for the day", before I manage to turn the damned thing off.
Like those formless creatures who lurk under our beds in the night, dreadful thoughts now writhe in the minds of sentient Yankee fans.
I put the car in park and sobbed, afflicted by whiplash, the caterwaul of car alarms, and dreadful thoughts of vehicular homicide.
During the night the demons and dreadful thoughts come, say the patients, and they become anxious and sad and start thinking why has this happened to me...and you have to listen and take your time (I 19).
Los Angeles, wrote Aldous Huxley in a 1926 essay in the anthology titled "Los Angeles: A Rhapsody," was a "city of dreadful joy," where "thought is barred" and "conversation is unknown".
