Literally.
For example: Scalecaller Peak because of the skeevers that shoot toxic stuff when defeated. And the first boss who's name is Orzun the foul-smelling.
This can be based on the dungeon's environment (how it looks), npc abilities, monsters, bosses, and mechanics.                
                                  
        Which dungeon is the most toxic 31 votes
        
                            
                            
                            
                    Fungal grotto
                    
                                            
                            
                            
 2 votes
                            
                            2 votes