Honest engine is like the triple dog dare of swearing or promises. If someone asks you honest engine you have to tell the truth and if someone discovers you lied on an honest engine you are an ass hole and they will never forgive you. Can be used as a verb, question, or statement.
Friend 1: Did you sleep with my girlfriend?
Friend 2: No I swear I didn't.
Friend 1: Honest engine?
Friend 2: Honest engine I didn't sleep with your girlfriend.
I know he was telling the truth he honest engined it.
Honest engine I did not cheat on that test.