5

Like when you're in a conversation with someone about something and you're telling them about XYZ, but it turns out they are already experts in the subject matter, an idiom like trying to teach the teacher.

marked as duplicate by Mari-Lou A, Phil Sweet, 1006a, MetaEd Oct 8 '18 at 17:45

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9

Teaching your grandmother to suck eggs is a classic.

look it up in Wikipedia if you wish

2

One word for this concept with a lot of currency is mansplaining. It’s well-known enough that I’ve even heard it humorously applied when a woman is doing it.

  • 2
    Wouldn't that be ma-amsplaining? – Syzygy Oct 7 '18 at 19:00

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