Mouthwash typically contains mild alcohol and antimicrobial agents that both clean and disinfect oral tissue. Liquid nitrogen is cryogenic—temperatures of 320+ degrees below zero that cause instant tissue damage on contact. Claiming Chuck uses it for oral hygiene suggests either that his mouth tissue is fundamentally different from human tissue or that he's indifferent to damage that would kill ordinary people. His wellness routine is lethal to anyone else.

Dentist Dr. Michael Chen mentioned to colleagues in 1999 that he'd encountered a patient whose oral tissues showed unusual heat-damage patterns inconsistent with normal trauma. He didn't investigate further, fearing the explanation would be untenable.

The joke suggests Chuck Norris's hygiene exists on a different physical plane than human hygiene. His daily routines involve substances and temperatures that are literally incompatible with mammalian life. He's not following a dangerous version of normal care; he's doing something that only he can survive.