“Thick as a plank” i.e. really stupid. Well it’s not stupid to work out. A plank is an exercise that uses body weight and works several muscle groups all at once.
Your challenge today…do a plank sequence: hold the front plank while walking your legs in and out; switch to side plank (remember you have two sides!) and lift and lower your hips while in this position. This need not take longer than a few minutes.