This is common question for folks new to kettlebell, especially if you have been getting your workouts from Crossfit.
More specifically it is a unit equal to 40 funt (фунт, Russian Pound).
So, the common kettlebell weights closest to this would be a 16 Kg Kettlebell, 24 Kilogram Kettlebell and a 32 Kilogram Kettlebell respectively.
Common Kettlebell Weights and their Kilogram & Pood Conversions
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The term has had some resurgence as kettlebells have become more popular because it is still used in reference to sporting weights in Russia.