take one's name in vain

1. To call upon (God) as a witness to your truth or honesty when you are lying; swear by (God) untruthfully.
You shall not take the name of the Lord your God in vain.
2.  {informal}
To talk about a person or mention his name.
"Did I hear someone taking my name in vain?" asked Bill as he joined his friends.
Categories: God friends {informal} {v. phr.}