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 takes form as universal armor which triggers against most moves that deal low knockback. Tough Guy's effect is most apparent when Bowser has low damage. While it is present in almost all of Bowser's animations, it is somewhat amplified by crouching, but not while crawling or attacking. As Bowser's damage increases, Tough Guy decreases. Different attacks have varying thresholds for how much damage Bowser can take before he can no longer "tough out" said move.
Tough Guy resists knockback up to 19 units. However, it was indirectly buffed in Ultimate due to Bowser's higher weight and the universal knockback reduction in all rapid jabs.
List of attacks affected by Tough Guy