Tough Guy is a passive fighter ability exclusive to Bowser. It allows him to resist flinching from weak attacks. This mechanic was first introduced in Super Smash Bros. 4, and carried over to Super Smash Bros. Ultimate.
Tough Guy is a form of knockback-based armor that triggers against attacks that deal 19 units of knockback or less. This is present in all of Bowser's animations, even when buried or frozen. This notably includes crouching, which thanks to the 0.85× knockback reduction it provides, improves the armor. Tough Guy is also present during attacks that utilize Bowser's armored smash and tilt attacks in Ultimate, giving him effectively two layers of it.
Because of the armor being knockback-centric, Tough Guy's effectiveness directly correlates to his percentage. As Bowser takes more damage, he takes more knockback, which gives Tough Guy a hard limit in every matchup. Additionally, effects that increase knockback, such as rage, will also affect Tough Guy. For a character-specific example, Hero's Psyche Up applies a 1.2× knockback multiplier directly onto his moves which, as a result, significantly decreases Tough Guy's effectiveness until it goes away.
Tough Guy was indirectly buffed in Ultimate due to Bowser's higher weight and the universal knockback reduction in all rapid jabs. The addition of damage-based armor into his moveset also assisted with this, adding a degree of safety at tighter percent ranges. As a result, it's not uncommon to see Bowser armor through rapid jabs until very high percentages and subsequently punish them.
Due to Tough Guy's very low knockback threshold, its usefulness is highly specific to the point of being near-ineffective in many matchups. While characters like Sheik will tend to struggle, others such as Donkey Kong are virtually unaffected.
List of attacks affected by Tough Guy