The phrase "frightful bore" is commonly used to describe someone or something that is incredibly tedious and uninteresting. However, there are several synonyms that can be used to convey the same sentiment. One option is "tedious to the extreme," which emphasizes the unbearable nature of the situation or person. Another possibility is "mind-numbingly dull," which suggests a lack of stimulation or excitement. "Monotonous" is also a useful synonym that emphasizes the repetitive, unchanging nature of the situation. Finally, "yawn-inducing" is a more informal synonym that conveys just how boring and uneventful the situation is.