Have you ever heard of fossil words? These are words that used to be used in normal spoken English, but now mostly only exist inside of idioms. Like dinorsaur fossils which are only found inside of rocks or pieces of amber, these words are mostly only ever used when using these idioms.
Here are some common fossil words and the idiom they are fossilized inside:
Idiom: let bygones be bygones
Idiom Meaning: Forgive past conflicts and reconcile.
"It's been 20 years since you had the fight! Let bygones be bygones and move on."
Idiom: run amok
Idiom Meaning: To behave wildly or dangerously.
"We can't just let the kids run amok!"
Idiom: good riddance
Idiom Meaning: Expresses relief at being rid of something or someone unwanted.
"Our noisy neighbors are finally moving out. Good riddance!"
Idiom: to and fro
Idiom Meaning: Constantly moving backwards and forwards.
"During the earthquake, the chandelier swung to and fro above the party."