It comes from the prefix M-D and it's easier to say than MD-80 on the radio. It sounds juvenile, but for example, if Ground asks you to hold short of a taxiway to let an MD-80 pass, it's easier and quicker to respond, "Ok, we'll give way to the mad dog," than to say, "Ok, we'll give way to the MD-80." That's the only reason, and most pilots who commonly refer to MD-80s as mad dogs with any regularity are Atlantic Southeast pilots in ATL who must yield to Delta's MD-88 fleet, and also the ASA pilots in DFW who must work with both AA and DL MD-80s daily. Other than on the radio, I've never, ever heard an MD-80 aircraft referred to as a "mad dog". It's just a playful use of the MD prefix.