'Zat You, Santa Claus?
Sure is dark out, ain't the slightest spark out
'Pon my slackened jaw!
Who's there? Who is it stopping for a visit?
'Zat You, Santa Claus?
A slurrish contraction of 'Is that so'? Used more commonly in the eastern US states, particularly New York and the New England region. It's a bit on the sarcastic edge.
Donny: I'm the baddest smartest and best looking superstar in this school. Chicks dig me.
Eddie: Zatso?