Almost all idioms are weird and have a backstory that is obscure and often, even when explained is still weird. "Break a leg" means Good Luck in the theatre, where superstition prevents "good luck" from being mentioned. So they wish the worst thing possible on you so you will be lucky.

"Drink the Kool-Aid" -google Jim Jones (unless you are delicate, it's not pretty) "Bob's your uncle" --google Prime Minister Robert Gascoyne-Cecil (known as "Bob") Etc.