boor / bore
A rude person versus a dull one, or the verb bore.
A boor is rude and coarse. A bore is tedious. To bore is also to weary someone, or to drill a hole. Do not write boor for the dull guest, or bore for the rude one.
Examples
Use He was a boor at dinner.
Use The speech was a bore.
Avoid He was a bore at dinner. (If you mean he was rude, not dull.)