What is the figurative meaning of "cold pizza"?

The majority of these usages are literal. If you had pizza, leftover from last night, you eat it cold in the morning for breakfast. It's not so much a metaphor as it is symbolic/representative of a unhealthy, undomesticated lifestyle. It means you're not eating a "real" breakfast, and you aren't even bothering to reheat the pizza. You're probably a hungover bachelor (or bachelorette).

It is a tasty meal, usually comprised of leftover pizza from the night before. However some people are known to deliberately order or cook too much pizza for consumption in one night and intentionally save some in the fridge for the next day.

As well one can argue that pizza (with or without beer) has all the major food groups. At least if you have tongue in cheek.

Last, some people I know develop a taste for cold (refrigerated) versions of other Italian-American foods, such as cold lasagna.

An Elephind newspaper database search for "cold pizza" yields 196 matches—193 from the United States and 3 from Australia. The vast majority of the results are from college newspapers, which makes sense because most mainstream, general-circulation daily newspapers later than the early 1930s are excluded from the search results (for copyright reasons). The decade-by-decade matches for "cold pizza" work out as follows:

1950–1959: 5 matches

1960–1969: 10 matches

1970–1979: 25 matches

1980–1985: 45 matches

1990–1999: 28 matches

2000–2009: 76 matches

2010–2019: 7 matches

Notably, not one of these matches involves the use of "cold pizza" in a figurative sense or as a description of a person or activity. In every case case, "cold pizza" refers to pizza that is cold—either because the initial fresh-from-the-oven heat has dissipated (a process that takes less than 20 minutes, owing to the thinness of the typical pizza) or because the pizza is leftover and has either been stored in a refrigerator or left on a plate for hours (usually overnight).

References to "cold pizza" have evolved in several ways. First, the earliest Elephind matches for "cold pizza" seem to involve the cooled-off-from-its-original-warmness sense of the term—and all appear in the context of criticism. The oldest match is from an advertisement for Sam's Pizza Kitchen in the [Champaign, Illinois] Daily Illini (November 9, 1956), which reads simply:

Cold Pizza is for the birds! All Pizza delivered hot!

The earliest instance of an article referring to "cold pizza" as a dining option consisting of one or more slices left over from a meal eaten hours or days earlier is from Erma Bombeck, "Call It Pillow Talk," in the San Bernardino [California] Sun (November 22, 1971):

Notes pinned to the pillow of a mother who has flu — by a well-meaning husband who has inherited the house and kids.

...

Dear Doris: Why in the name of all that is sane would you put soap chips in the flour cannister! If you have time, could you please come up with a likely spot for Chris' missing shoes? We've checked the clothes hamper, garage, back seat of car and wood box. Did you know the school has a ruling on bedroom slippers? There's some cold pizza for you in a napkin in the oven drawer. Late tonight, Driving 8 Girl Scouts to tour meat packing house!

The concept of "cold pizza" for breakfast debuts in Elephind's newspaper database results in Clare Brown, "Clare's Column: Stack Up a Hearty Breakfast," in the Clare [Michigan] Sentinel (September 18, 1974):

Nowhere does it say breakfast has to be toast and eggs or cereal and milk, although these are excellent breakfasts, if these things turn kids off, try a hamburger deluxe, a bowl of soup, or a slice of cold pizza and milk. They fit into the good breakfast criteria just as well as our sterotyped concepts of what makes a breakfast.

The earliest match for "cold pizza [for breakfast]" as a representative lifestyle dietary choice for single men, college students, indie band members, computer geeks, etc., appears in "Magnetism plus equals Starsky & Hutch," in The Australian Women's Weekly (September 15, 1976):

The two of them seem to be the toughest and fittest plain-clothes detective commandos yet to grace TV's Los Angeles police force.

Maybe their breakfast helps them. In a recent episode Starsky had a breakfast of cold pizza and root beer (a rich, dark-brown soft drink).

From these origins, "cold pizza" has emerged as a cultural marker in roughly the same vague yet tendentious way that "brie and chablis" did (briefly) in the 1980s. Here is a typical instance, from Cheri Monaghan, "Going Home: Those Visits with the Parents Who Remind You to Wake Up in the Morning and Give You Gifts of Dog Halt," in the [University Park, Pennsylvania] Daily Collegian (February 7, 1986):

Remember the joy of newfound freedom away from screaming siblings and prying parents; staying out all night, cutting classes for days on end, playing the stereo at decibel levels only dreamed of when still at home, eating cold pizza and Pepsi for breakfast and doing a dozen other things our parents were likely to drop dead if aware of? At last we were really living!

And from Kevin Padrez, "It's OK, He's Just a College Student," in the Stanford [California] Daily (June 15, 2007):

After graduation I don't expect my lifestyle to change very much. I'm still going to eat cold pizza for breakfast and buy my formal wear at Savers. The only difference is I will now be considered a dead-beat twenty-something, not a typical college student.

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Conclusion

In the United States, at least, "cold pizza" has not emerged as a widely recognized figurative expression. Instead, its force is as a shorthand representation of a particular (usually college-centered) dietary lifestyle. That the term retains this status almost half a century after its "it's what's for breakfast" debut is a testament to the somewhat surprising continuity of post-high-school eating habits, as channeled by young adults and as attributed to them by older ones.