A dash more bald than others of its kind?
No, the fact is that no one knows for a certainty how the word “balderdash” originated.
When, in the late sixteenth century, it first appeared in print it referred apparently to a light frothy or bubbly liquid, “barber’s balderdash” was the term used.
Not long after, in 1637, John Taylor, the “Water Poet,” said that beer mixed with wine was called balderdash.
Beer and buttermilk, said Ben Jonson, is “balderdash.”
Thus anything frothy, bubbly, or impossibly mixed, whether liquid or language, became so termed.
As to origin, it may have been formed from the slang balductum, a hundred years the elder but itself of unknown ancestry, which also met Jonson’s definition.
This is pure speculation, but there is a Norse god named Balder, who is associated among other things with wisdom. Could it be related to that, as in the word being a sort of plea to Balder to “dash” something nonsensical?
e.g.
May-Balder-Dash-This ????