〜といえども
even though, even (someone who is)
Noun / Plain form + といえども
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といえども means "even though it's X, even X": 子どもといえども、ルールは守らなければならない, "even children have to follow the rules".
The first half names someone or something you'd expect to be an exception: a child, an expert, a joke, a single yen, a holiday. The second half says the general rule still applies, often with なければならない, ことはある or できない.
It's a formal, written relative of とはいえ (N2) and でも. It's common in speeches, essays and rules. With a minimal amount, it means "not even": 一円といえども無駄にはできない, "we can't waste even a single yen".
Don't confuse it with といえば (N3), "speaking of".
Examples
子どもといえども、ルールは守らなければならない。
Even children have to follow the rules.
冗談といえども、言っていいことと悪いことがある。
Even as a joke, there are things you shouldn't say.
一円といえども、無駄にはできない。
We can't waste even a single yen.
専門家といえども、間違えることはある。
Even experts make mistakes.
休日といえども、彼は仕事のことばかり考えている。
Even on his days off, he thinks about nothing but work.