A "workout finisher" is a short and intense exercise or set of exercises that you do at the end of a workout to really push and challenge yourself. It is meant to be the final part of your workout to complete your training session. Full definition
The Ultimate Full Body Conditioning Workout finishes with a Kettlebell Mountain Climber Deadlift to empty the tank and push you to your limit! (onnit.com)
Farmer's walks are one of the best workout finishers, period. (fitnessandpower.com)
In Heath's experience, dips can be a great workout finisher after a heavy triceps routine. (fitnessandpower.com)