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1.
The Doctor lies. (from Doctor Who)
Amy: But he (the Doctor) was dead.
River: Who told you that?
Amy: He did.
River: Rule one: the Doctor lies.

River: He said no one could save him. But he must have known I could.
The Doctor: Rule one: the Doctor lies.

River: Okay. I'm going to tell you what I probably shouldn't. The Doctor's last secret. Don't you want to know what he whispered in my ear?
Amy: He whispered his name.
River: Not his name, no.
Amy: Yeah it was. He said it was.
River: Rule one?
Amy: The Doctor lies.
by atalienay October 12, 2011
 
2.
Literal translation is 'don't be a pussy'. Most often used when berating someone with 'you forgot rule one' implying that if one has forgotten rule one then they're indeed a pussy.
Dan: I'm a little tired, my boots are chaffing and the wind is blowing in the wrong direction.
Gora: You're in violation of rule one. Get over that kicker.
by Dangleburt Humpledink August 05, 2010
 
3.
The Doctor lies.

Most of River Song's "The Doctor lies" is about it being her own rule #1 for figuring out what's going on when things don't seem to make sense. If things seem incomplete or impossible, consider first if The Doctor lied about something.
Rory Williams: Why did he tell us he was dead?
River Song: Rule One: The Doctor lies. So do I.
by 10DoctorWhoFangirl10 June 10, 2011
 
4.
Rule one refers to the first rule of deciding if someone is worth getting involved with: "Never put your dick in crazy". Other more gender-neutral ways of phrasing this rule such as "never invest in crazy" have been suggested, but the central meaning stays true regardless of phrasing.
A:"Oh man, do you think I should get with him?"

B:"No, chicka, he breaks rule one."

A: "Okay, not happening then."
by Owlsongbitter May 21, 2014
 
5.
A rule that states anything that results in any form of labor or risk should not be taken unless the amount of money your offered is to your liking
Only fight when you have been paid to do so

Never take a Favor...you get no money out of it

Everything....and everyone has a price....

This is Rule one
by Dragyn21 April 19, 2010