Acting up, a peculiar phrase. It's what people say to minimize the gravity of their condition. It implies that the offending part (heart, stomach, liver, whatever) is a fractious, bratty child, which can be brought into line with a slap or a sharp word. At the same time, that these symptoms--these tremors and pains, these palpitations--are mere theatrics, and that the organ in question will soon stop capering about and making a spectacle of itself, and resume its placid, off-stage existence.