Pull your finger out.
“I can still see my parents coming home from parents’ evening with their hair falling out, having been told once again that I was going to end up as a tramp on the street corner. It wasn’t until I was 14 that I suddenly realised that if I didn’t pull my finger out I really was going to be a tramp on the street corner.” — Richard Madeley, Daily Mail, 18 Oct 2014
In other words, stop faffing around and get on with whatever you need to get on with …
There are various theories about where the phrase originated — including that it was RAF slang, it referred to courting couples, and that it was a nautical saying about crew members loading cannons with powder. Read some of those suggestions in this Guardian notes and queries piece from 2011.