Where does mad dog come from?
In the 1980s, mad dog emerged as slang for a "thug," shifting to senses of "rebel" or "threatening stare" in the 1990s. The underlying senses of the term characterize someone as "acting vicious or crazy" like a mad dog—with mad here preserving an older and original sense of "rabid," the effects resulting in aggressive behavior. The connotation of mad dog softened in the 2000s, often applied as a moniker or nickname for someone positively considered "ferocious."
The 1993 comedy Mad Dog and Glory, featuring Robert De Niro, Uma Thurman, and Bill Murray, follows a crime scene photographer dubbed Mad Dog. Mad Dog Sports Radio, as another notable example, started broadcasting sports news and talk on Sirius XM in 2008. The name suggests they are "wild" about sports. Elsewhere in sports broadcasting, Christopher Mad Dog Russo co-hosted a very popular sports radio show, Mike and the Mad Dog, in the 1990s and 2000s. Another prominent Mad Dog is James Mad Dog Mattis, the Secretary of Defense under President Donald Trump. Mattis is said to have earned the nickname while serving in the Gulf War, apparently as he was a formidable soldier.
As a slang term for cheap wine one buys simply to get drunk, mad dog takes off in the 1970s in reference to MD 20/20, a brand of inexpensive fortified wine from Mogen David wineries—hence the abbreviation MD, which apparently people joked stood for Mad Dog due to its inebriating effects. This mad dog harkens back to the 16th century, when mad dog was a term for a kind of strong ale, the name likening the ensuing intoxication to the behavior of a rabid dog.
Who uses mad dog?
The "crazy" senses of mad dog are commonly seen as complimentary and "macho" nicknames in sports, military, and law-enforcement contexts. Many popular media titles also use the term for this meaning, such as Amazon Prime's Mad Dogs (2015–16) and the South Korean crime series Mag Dog (2017).
The "cheap wine" mad dog is found in informal speech and writing especially from middle-aged to older adults. Popular websites often refer to MD 20/20 as Mad Dog or Mad Dog 20/20 when writing humorous articles about getting extremely wasted on a budget.
Mad dog, or "threatening stare," has inspired a slang verb increasingly found in colloquial speech and writing on social media in the late 2010s, to mad dog, or "glare at someone in a mean or judgmental manner."
I'm printing like a mad dog! Go press, go!
@geekbert, December, 2007
Some waifish, subscription menswear box-wearing developer nerd just walked by my window and mad dogged me while expelling an enormous vape cloud and that’s pretty much 2018 in a nutshell
@danieltaylor, April, 2018
Like stereos or fine wines, some appreciate quality and we call them connoisseurs. That being said, some people just want to crank up the music really loud and get drunk on Mad Dog 20-20.
The Chairman, ScubaBoard (forum), March, 2012