The Default Denial is a debating tactic that is usually expressed in the form "What's stated without proofs can be dismissed without proofs.", but what it's actually intended to mean is: "What's stated against accepted knowledge can be denied without proofs."
This concept is flatly refuted by the most elementary laws of logic. In logic, every denial is a statement in itself: to deny something means to state the opposite of that thing, and therefore, the burden of proof lies on the denier as well as the claimant. Since no statement is ever proved "a priori", to claim that something can be denied without proofs is the same as saying that *anything* can be denied without proofs.
Similarly, by the law of double negation, every statement is the denial of its opposite, so to say that anything that's not proved can be denied without proofs is the same as saying that everything can be stated without proofs, making the very concept of "proof" utterly useless, and logic itself together with it.
In order to support this slogan, the example of a believer refusing to prove its faith is often invoked, implying that the believer must prove the existence of God while the atheist doesn't need to prove anything; but for the reason explained above, that is flatly wrong: in order for their positions to be taken seriously, both the believer and the atheist must justify them, because both of them are making very clear non-trivial claims. In logic, the only statements who are lifted from the burden of proof are those who are omnicomprehensive (i.e.: tautological), since they`re true a-priori; as is the case of the agnostic, who neither affirms nor denies the existence of God.
Aside of its intrinsic validity, this slogan is often dishonestly used as a way to subtly imply that the opponent has never proved his own statements without explicitly saying it, in order to shift the focus of the discussion away from the statement itself and avoid those proofs from getting attention. In other words, this argument's real intent is not to make some point about logic or argumentation, but just to lazily belittle the opponent's position while attempting to sound smart in the process.