Uber Evil said:
Figures of speech aren't really meant to be understood, just acknowledged.
Pretty much this.
On the one hand, you say you "don't understand" the "from the bottom of my heart" idiom, but that's clearly not true - you definitely understand what someone means when they say it.
The thing you don't understand about the saying isn't its meaning, it's the the compositionality. As it turns out, a
lot of language is incredibly problematic if you want to be able to speak about it entirely compositionally, not just idioms.
Sayings like these are pretty great examples though. My favourites are where repitition has lead to accidental reversals of the saying becoming more common than the original, making it not just compositionally weird, but seemingly completely contrary to the actual meaning of the saying.
Two of the most common ones (I've seen a few others in this thread too):
"I could care less."
"A bird in the bush is worth two in the hand."
Also, this thread is
rife with folk etymology. You should trust approximately none of it. And again, these things aren't
supposed to make sense compositionally. When it comes down to it, the fact that most language seems to be mostly compositional is by most estimations pretty accidental.