For whatever reason, some characters can ignore another character's powers, be it offensive or defensive.
They might be immune to every kind of magic, or their abilities are "too powerful" to be resisted but whatever the case, other characters' powers don't work on them and their own powers are not impeded in the least. If heroes attempt to use The Worf Barrage, they'll step out of the Smoke Shield and casually dust themselves off. When the Barrier Warrior tries to block their attacks with a Beehive Barrier, they'll break it into hexagons and knock them out. It can also take the form of a wrestler using their ultimate move and eliciting little more than a raised eyebrow from their opponent.
They aren't canceling their opponent's powers (not actively, at leastnote ), and they often aren't explicitly stated to be immune or resistant beforehand. They're just that good. It's almost as if it were a make believe children's game where one of the kids refuses to "play by the rules" and insists they're invincible and immune to their playmate's imaginary powers. That's impossible... Right?
Depending on execution, this can be a terrifying reminder of exactly why they're called the BIG Bad, or a very groanworthy way of adding Fake Difficulty for a hero. If a villain has benefited from No Sell for the first four acts, by the time the hero masters his powers/confidence in the fifth, it's likely that he'll be the one using No Sell along with a Super Mode or Heroic Resolve.
The trope's name is a term used in Professional Wrestling circles. In all types of acting, "selling" means an actor reacts as if he had been hit hard when the attack didn't make contact or was harmlessly light. Professional wrestling refers to it as a "no-sell" when the wrestler that was struck doesn't react to the hit (i.e. he just stands there, as if his opponent were punching a brick wall). Originally, this was usually a case of the wrestler taking the hit just being a dick, and was highly frowned upon because it was seen as undermining Kayfabe. But it also came to be used as a way to demonstrate that a wrestler is just that tough. Some wrestlers use no-selling as their main schtick.
Immunities to specific attacks and/or Status Effects are almost always present in Tabletop Games and are the main purpose of Damage Typing. As a game model grows, it's more and more likely to get into Lensman Arms Race of effects, immunities and immunity-breaching effects. See also The Law of Diminishing Defensive Effort.
A favorite of the Implacable Man and The Juggernaut. May cross with The Determinator. inversion is No Saving Throw.
When a video game boss can do this, it is often a Hopeless Boss Fight. An example where the boss can do this at first but a cutscene allows the hero to damage them is a Seemingly Hopeless Boss Fight. Attack Its Weak Point is where the boss no-sells everything except to one spot.
Compare to:
Anti-Magic: When they can block off someone's magic.
Balls of Steel: When this is applied to a Groin Attack.
Contractual Boss Immunity and Useless Useful Spell: When this is applied to video games.
Cross-Melting Aura: An especially evil creature no-sells their Weaksauce Weakness to holy items.
Disability Immunity: When a character is immune to one's power because they cannot perceive/process the manipulative signal.
Feel No Pain: When the no-sell is the result of the attack not being felt by the target.
Immune to Bullets: When the character can no-sell Five Rounds Rapid.
Immune to Fire: When the character can no-sell being set of fire.
Immune to Flinching: When the character actually is getting injured by the attack, but you wouldn't know that by looking at it.
Immune to Mind Control: When attacks on the mind can be blocked off.
Kung Fu-Proof Mook: When mooks do this.
Nigh-Invulnerability: When they can no-sell almost everything.
Psychic Block Defense: When someone is immune to mind reading.
Punch! Punch! Punch! Uh Oh...: When the attacker uses hand-to-hand attacks instead of special powers.
Resistant to Magic: When a character resists (or is unaffected by) magic.
Shooting Superman: When this crosses over with Idiot Ball. Usually, this is accompanied by a Finger Wag or other kinds of taunts.
Too Dumb to Fool: When a character is too stupid to be affected by things such as Mind Rape or attempting to break him by talking.
