"A penny" isn't an idiom - it's just a one-cent American coin.
"Penny pincher" is the slang term for a stingy person.
"Penniless" is not an idiom. It means that you don't have a penny to spend. It's used as an exaggeration to mean that you don't have any money.
It's TO SPEND A PENNY. It means to urinate. In the old days, public toilets cost a penny.
The German idiom der Groschen ist gefallen equates to the English idiom the penny drops or the penny dropped meaningA belated realization of something after a period of confusion or misunderstanding.
penny for your thoughts
A bad penny is an idiom for something which is unpleasant or unwanted, especially something which appears at an undesirable time.
It means to urinate. At one time, public rest rooms cost a penny to use.
It is not an idiom, it means your nose is itching.
It's not really an idiom. It means "what are you thinking about."
RFP is not an idiom. It's an abbreviation.
"Sieve" is not an idiom. See the related link.
It's not an idiom. It means the tip of your nostril.