A person who behaves in the manner the stereotypical dad would. This includes, but is not limited to, Hawaiian Shirts, dead memes, and the regular dad joke.
One need not be a dad to earn the title of "The Dadliest."
Paul: Guys, let's goget some food. I'm really hungry.
James: OH, NICE TO MEET YOU, REALLY HUNGRY! I'M JAMES! *dabs*
Tom: James, you truly are the dadliest of them all.