It comes from "my bad" when you make a mistake, "my dumb" is when you did something dumb.
Shannon "No, it's not me it's the other Shannon."

Chad "Oh sorry, my dumb!"
by Zilla December 13, 2004
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national hit your dad with a bug day
i meant to say mug but that makes it even more dumb
please don't take this seriously
do not hit your dad with a bug
Here's my dumb*** holiday
national hit your dad with a big day
you mean bug?
yes
by Orrinpants November 01, 2021
Get a Here's my dumb*** holiday mug for your daughter-in-law Julia.