It's a common phrase, but have you ever wondered what it really means? Undermining your elders is when you, as a youngin', try to sabotage your elders' attempts to tell you what's what.
Example:
Grandma: "You should wear this sweater, it's warm." You: "No, grandma, I'll just wear my shorts, I'm a rebel."
It's like, you're not just disagreeing with their fashion sense, you're actively working against their will, their wisdom, and their ability to remember what they did last week.
But don't worry, it's just an idiom, so you're not actually undermining anyone. Unless you're a teenager, then you're probably doing it all the time.
Back to Idiomatic Expressions 101
Or, if you're feeling brave, try Undermining Your Parents
Or, if you're a rebel, go straight to Undermining Your Teachers