Either that or wrist curls when done like this.
“Not too many people do one-arm dumbbell wrist curls, although Dave Draper and Casey Viator used to do them, and both men had massive forearms. Sit at the end of a flat bench or on a short stool. Hold a moderately heavy dumbbell in your right hand, and anchor your right elbow at the top of your right thigh beside the crotch and near the hip bone. Try different spots in that general area until you find the one that feels best and allows you to best contract your forearms. Use your free hand to support the working arm and to help yourself do forced reps. If you do the exercise properly, you’ll experience a white-hot burn in the upper underside of your forearm. You’ll actually see that part of your forearm bulge with every rep. I’ve found it to be a real size builder, especially when used in conjunction with EZ-curl-bar reverse curls, hammer curls and regular wrist curls”