a) What does "to coin a phrase" mean?

b) This sentence...>>>RLC points out that the phrase 'hang up' is always untrue, because things can't hang up - by definition, they hang down. That's not an untruism, it is an oxymoron. But interesting. is way off.

Telephones used to hang on the wall, and the cradle to put the listening piece was higher than the individual so they'd have to hang the receiver up to disconnect.

Phone booths still have receiver that have to be hung up.