When someone always has to one up you in every aspect of life.
Person 1 "There's a massive cat in my back garden!"
Person 2 "Well there's a Panther in mine!"
"I drank like half a litre of vodka last night"
"Yeah I had a crazy night, I drank a whole litre"
"Last time I went to the dentist I had two fillings"
"Oh that sucks for you, I had four last time I went in"
Person 2 panthering person one.