That means stinky finger. You know, as in the finger?
SPD boss Sigmar Gabriel recently got some Sommerloch good press by doing what is generally considered to be a bad thing. Only he was flipping off neo-Nazis who were heckling him so that made it a good thing.
Getting in a little more mileage out of it, he has now said in an interview that his only regret is not having used two stinky fingers (no, not the peace sign, he means two hands).
“Ich habe nicht beide Hände benutzt.”
PS: A Sommerloch or summer hole is the “silly season” during summer vacation months typified by frivolous or unimportant news stories in the media.