Un boulet is a heavy ball, usually a cannonball or an iron ball tied to the foot of a prisoner.
Coloquially, the word has a logical figurative meaning: a useless person who prevents others from going forward, or an annoying person, for example someone who makes unfunny jokes or doesn’t understand that you don’t want to talk to him. Or, more simply, an idiot.
Quel boulet, ce mec ! Il m’a suivi pendant toute la soirée, j’ai pas réussi à m’en débarrasser !
What an annoying guy! He followed me during the entire evening, I couldn’t get rid of him!
Mon frère était censé venir me chercher à la gare mais il a oublié, ce gros boulet.
My brother was supposed to pick me up at the train station but this idiot forgot.
Boulet is mostly used to talk about men. I have sometimes heard a less common feminine variant, boulette, which normally means “little ball” or “dumpling”.
Listen to the examples: