also spelled "tuffdog", refers to the quality of a person holding either the leading position or is a probable winner to a competitive game, regardless of its format, wether in video games, card games, board games, etc. It is commonly used as a compliment to another gamer, and may even be used in the self-congratulatory sense.
May also be conjugated accordingly
"Dude, you've been toughdog all game!"
"Wrong, I've been toughdogging my whole life."