A warning given when someone tries to talk around someone on a certain subject, but the person is very familiar with the subject matter.
Employer: It seems you tested positive for marijuana.
Employee: I was at a party and people were smoking around me.
Employer: The party must have been in a Mini Cooper with the windows rolled up to have this high of a concentration in your urine. Don't bullshit a bullshitter. Clean yourself up or you're out.
The words of wisdom received when a person tries to deceive someone who is good at deceiving people.
So I told my dad "I drove your car to the party and was drinking, but my friend who wasn't drinking drove your car home." My dad said, "Son, I'm a used car salesman, Don't bullshit a bullshitter."