“Six of one, half a dozen of the other”
What it means
Two options that amount to the same thing.
Where it comes from
Half a dozen is six. The phrase states an identity while pretending to offer a choice, and its comedy lies entirely in the arithmetic being obvious.
How sure are we?
Attested from the early nineteenth century, first recorded in a naval context. The dozen was so embedded in trade that expressing six in two different ways would have felt entirely natural.