Not a drink. A Swedish highball is when someone waves their hand high over their head to say hello.
It comes from a railroad term, 'highball'. A highball was a railroad signal that looked like a ball on an arm on a tower. When it was raised, it meant a clear track ahead. A trainmaster or brakeman could hold their arms high and move their hands in a circle for the same meaning. The Swedish farmers were known to wave an arm high over their heads and shout "hej" (hello) when greeting someone.
Gust was in the field when he saw Ole on the road. He gave him a Swedish highball and shouted "Merry Christmas."