bring a knife to a gunfight

To come poorly prepared or equipped for some task, goal, competition, or confrontation. Often used in the negative as a forewarning or piece of advice. Make sure you have researched your position thoroughly and comprehensively before the debate. You don't want to bring a knife to a gunfight. If you haven't studied law, representing yourself in court is like bringing a knife to a gunfight. That skinny twerp is bringing a knife to a gunfight if he thinks he can best me, two-time champ, on the wrestling mat.