Quick Google search to the rescue!
In professional wrestling, a heel (also known as a rudo in lucha libre) is a wrestler who portrays a villain, "bad guy", "baddie", "evil-doer", or "rulebreaker", and acts as an antagonist to the faces, who are the heroic protagonist or "good guy" characters. Not everything a heel wrestler does must be villainous: heels need only to be booed or jeered by the audience to be effective characters, although most truly successful heels embrace other aspects of their devious personalities, such as cheating to win or using foreign objects. "The role of a heel is to get 'heat,' which means spurring the crowd to obstreperous hatred, and generally involves cheating and any other manner of socially unacceptable behavior."
Heel (professional wrestling) on Wikipedia.
Bulbapedia also says this:
This Pokémon disregards the safety of its opponents and on-lookers, sometimes striking the opposing Trainer with attacks.
It is a violent Pokémon with rough, aggressive, and egotistical behavior. It will sometimes ignore its Trainer's orders when it is not in the mood to listen.
While it tends to act like a heel, Incineroar is very happy internally when receiving the admiration of children or young Pokémon.
Incineroar.
Basically, it has do with the role of a heel in wrestling, and it doesn't have anything to do with feet.
Don't worry, I also didn't know what a heel was.
Hope I helped!
