Language works in wonderful and mysterious ways. Idioms are responsible for a decent chunk of this wonder.
To someone just learning English, Bob’s your uncle means something very different to what we understand that it means.
In becoming an idiom, the phrase has come to mean something beyond the literal sum of it’s parts.
Idoms are shared through place, but also shift through generations.
We don’t use ‘spend a penny’ to describe taking a leak anymore because we don’t use pennies, nor do we pay to use public restrooms.
For those unfamiliar with the weight categories in fighting, punching above one’s weight might not be a naturally obvious phrase.
And unless you’ve lived out bush, it’s unlikely that you’d ever understand why a cup of tea which is too hot was brewed with short sticks (it’s because kindling on a fire generates more heat that embers or logs).
Take time to catalogue the idioms you know, and the ones you’re at risk of letting go if any certain person in your life were to hit the road.