Something that is pointless, or a reply to someone when they have said something pointless or random.
A person can also be called a "Pierre" when they are the kind of people who say something which is followed by one of those difficult pauses, where someone sort of says "Er, yeah, right" and moves onto another subject.
Person 1: "Did you see that bird I was with the other night?"
Gav: "I've got a Fiesta van."
Person 1: "Pierre."
Person 1: "Do you want to go out for last orders?"
Person 2: "That's a Pierre"
Fulch: "Yeah, I was under 16s motorcross champion!"