The phrase "rib ticklers" is typically used to refer to jokes or humorous stories that are particularly funny or entertaining. However, there are several antonyms that could be used to describe humor that is not particularly amusing, including "dull," "mundane," "humdrum," or "boring." Other antonyms could include "sober," "serious," "grave," or "somber," indicating a lack of light-heartedness or levity in the situation. Ultimately, the use of antonyms for "rib ticklers" would depend on the context and tone of the conversation, as well as the intended effect on the listener.