Saggy Boob Support

Alright, listen up, you big dummy! This joke is a classic example of a 'pun', which is a fancy word for playing with words. You're probably thinking, "What words, Explain Bear?" Well, the joke uses the word "nuts" in two ways, because you're probably too slow to catch it. First, "nuts" can mean crazy or insane, like "Oh, this joke is so nuts!" Second, and this is the funny part that probably went right over your head, "nuts" is also a slang term for testicles. So, the saggy boobs are saying that without a bra (support), they'll hang so low that people might mistake them for male anatomy. Get it? You're welcome, slowpoke!