Hey kidgolfer,

So I've actually asked Casey this very question and this is what he told me:

Casey used to play hockey, as a goalie and his teammates gave him the nickname “bigdog”.

“pckt5s” comes from the fact that he won his first tourney, or first major tourney with pocket fives.

Put those together, with a little tweaking, and you get the screen name: bigdogpckt5s.