When it comes to humor, it can be subjective, and what one person finds side-splitting, another may not. However, if we're talking about antonyms for the word "most side-splitting," we could consider the following:
1. Dull: Something that is dull lacks excitement and interest, and therefore would not be considered side-splitting.
2. Boring: Similarly to dull, something that is boring does not hold one's attention and is unlikely to elicit side-splitting laughter.
3. Unamusing: If something is unamusing, it is not funny, and therefore it would not be considered side-splitting.
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