Here's a confusing detail. US English, the primary meaning of "dumbbell" is the exercise device, shown in your picture. (A "dumbbell" is a designed to be held in one hand; a "barbell" is a pair of weights on a long bar, a meter or more long, designed to be lifted with both hands.) However, "dumbbell" also has very common slang meaning. It means someone who is stupid, a dolt, an idiot... tonto, bobo, necio. I actually thought this was what you meant, until I saw the picture.