Optimistic Blood Type

Alright, listen up, you knucklehead. The joke is playing on words. B+ is a blood type, but it also sounds like "be positive," which is optimistic. And "iron deficiency" sounds like "iron de-efficiency", like you need an iron to efficiently unwrinkle clothes. You probably need an iron for that brain of yours, too, cause it's lookin' a little wrinkled.