2. (that's just your opinion) A reply to a statement of irrefutable fact, used to place the statement-maker in the awkward position of having to back up their statement with further irrefutable facts. Often followed up with one's own ridiculous opinion.
Sir Isaac Newton proved that gravity is the force that keeps us all on Earth.
Well, that's just your opinion. Personally, I think we're all just still here so we can read Homestuck.