Why are you saying something "for" yourself when your parent asks you what you have to say for yourself?

I was listening to a podcast today and heard someone mockingly ask the guest "Well, what do you have to say for yourself?". The conversation spun off in some other direction, but I momentarily expected the comedian being interviewed to pivot on the question and head into absurd grammar-land: "Who else would I say something for?"

Which made me wonder: why are we asked to say something for ourselves when we are having an explanation demanded of us? I can't think of any other expression where we say things for someone except when we are literally speaking for those with no voice: but clearly I have a voice myself. So why am I speaking for myself? How could it be otherwise?

Where did this idiom originate? Is it implying something in its formulation that isn't part of its current accepted meaning? I can't find much in my various searches, as Google returns only lots of people being interrogated and a hit on ell asking about "what have you got to say for yourself" vs "what do you have to say for yourself", without asking about the oddity of the idiom itself.

The closest thing I've found is a Google hit for a PDF paper implying that the phrase may somehow be related to the legal right against self-incrimination. But that seems very odd to me as an origin for an extremely common idiom.

Solution 1:

"for" can mean "on behalf of". It is not much of a stretch to use this to mean "what do you have to say in defense of yourself", particular if voiced with strong inflection, as if challenging. The implication is "I dare you to not defend yourself; you have been accused". But it's not really an idiomatic formation, it's just one of the dozens of meanings of "for".