It's when a girl is giving you a footjob with skates on her feet and you then cum super-hard as a result!
Bro 1: Dude, what did you and your Finnish girlfriend Anna do last night?
Bro 2: BROOO!! She gave me a Finnish Finish, it was sooo intense! I came right on her face!
*Fist-bump between the bros*
The result of sexual conduct with a Finnish girl. The act entails the Finnish girl vomiting at point of orgasm.
'Hey Jamie did you get with Jenni?'
'Yeah! She Finnish finished everywhere!'