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Tighty-whiteys. Yes there are two lines that look like wetness indicators, but it's decidedly not puffy, and there are no tapes, or even lines on the sides. What I'm wondering is why you'd think it's a diaper, when the joke is on the fact he's more or less naked.
I tend to like reading Dilbert comics, But I don't think it's a diaper. And I would agree with pojo, don't look to much into it. Why the hell would it be a diaper? There is very little in the article to suggest it's part of the joke!