I undertook arduous, minute-long searches in the Ngram viewer (nothing) and on the google for search strings

"that's a cold pizza"
"it's a cold pizza"
"he's a cold pizza"
"was a cold pizza"
"is a cold pizza"

I got lots of restaurant reviews (mostly negative, of course), a link to the lyrics of the song "Cold Pizza Warm Beer" by Gaye Adegbalola

It’s just the morning after, I don’t remember who was here.
I’ll eat a slice of that cold pizza, and wash it down with warm beer.

references to the sports talk show on ESPN2, and instructions for making a palatable breakfast. Possibly the song lyric is metaphorical. I was about to give up when I ran into this from an article in Time Magazine (August 29, 2011) entitled "Another Slice of 'Cold Pizza'? The Man Most Likely to Lead Japan" about then Japanese Finance Minister Yoshihiko Noda, who was expected to become Prime Minister:

Can Noda lead? Analysts are not sanguine. "He emerged as a compromise," says Jeff Kingston, Director of Asian studies at Temple University Japan. "He's not charismatic, or a populist or a good communicator, [and] not a particularly bold or visionary leader. He's sort of a 'Steady Eddy' and doesn't raise expectations that much. ... Says Yoshi Yamamoto, a political advisor to DPJ [Democratic Party of Japan] congressmen: "Noda seems a bit like the former LDP [Liberal Democratic Party] prime minister Keizo Obuchi [in the 1990s], generally known internationally as a 'cold pizza,' but respected domestically by working level staffers and officials as someone who listens and rewards."

I can find no uses for Urban Dictionary's claims for sexual and drug connotations.

I do not think that finding the earliest usage of “clock position” on the net will prove origin of the phrase. I have noticed that very often in history certain scientific ideas “spring up” spontaneously, almost simultaneously, in different geographical locations. This has become more common over time with the increase in availability of internationally published scientific works in related studies. However, tracing origins of ideas can also be difficult due to the habit that some scientists in the past had of jealously guarding their discoveries over years and even decades.

It can be inferred (but not documented by resources commonly available) that some early medieval astronomers, like many astronomers of today, relied on clock position to describe the arc of the moon through the night sky: the earliest astronomical clock faces were already in existence in the 14th century.

Chaucer´s Treatise on the Astrolabe (1326) is considered the “oldest work in English written upon an elaborate scientific instrument”, and comes very close, but not exactly to using clock positions. Although he generally used degrees of arc and Zodiac signs, he also mentioned a 24-hour system.

Much later, polymath Robert Hooke (b. 1635) was possibly one of the first to write about clock position using the 12 hour system in English. Hooke trained as a watchmaker, constructed microscopes and telescopes, observed tiny objects and the moon, and invented a semaphore system in 1691. It is entirely possible that he described features on the Moon using clock coordinates. This makes sense as the full moon never changes orientation, and is the same wherever viewed.

Due to his cantankerous temperament, Hooke was not well-liked by his contemporaries: some (such as Newton, with whom he entered into disagreement over gravity and inverse-square law) even went so far as to try and bury his existence after his death. Much of his work was published posthumously. It is speculated that some just disappeared.

So why is 12 up, or in front, and 6 down, or behind?

All sundials going back to Mesopotamia relied on the Sun being at its highest point at the 6th hour of the the 12 hour day. Romans used a 12 variable-hour day, but marked noon as the VI hour on a sundial that resembled an inverted airport radar .

Notice that equatorial sundials, aligned in parallel with the equator, were the first to translate noon, or the 6th daylight hour, to a more horizontal position.

At some point in time, already familiar with the use of astrolabes and equatorial sundials, astronomers probably influenced clockmakers who started marking the day in 24 hour cycles, with the 12th hour being midday. When mechanical clocks came into existence in Northern Europe and England, this was reflected with the 12th (noon) hour being in the highest position and up on a vertical dial. With gradual acceptance of the 12 hour face (as opposed to the “double 12”) this became the model for most clocks, and 12 was always up, and 6 down.

The invention of the pocket watch led to the concept of clock position and direction relative to the observer. At some point in time some astronomer pointed out that the pocket watch could be used to orient an explorer by aiming the hour hand at the sun and picking South out as being the mid-position between that and the 12. This was a reverse usage to the original theory of the astronomical clock.

It has been commented that flag semaphoring has relied on the positions of the hands of the clock to describe how to form letters and numbers since the 19th century. Military Signal and Boy Scout “field instructions” on flag semaphore bear this out.

But here things remained for a period of time. Ships, and later airliners, navigated using sextant, and magnetic headings based on North and South, and depended on the precision clock for longitude. Map readers on land did pretty much the same.

So when did it become useful to use phrases like “on your six” or “enemy at 12 o clock high”?

Probably not until two things happened at roughly the same time--fast maneuverable war-planes, and the invention of air-to-air radio communications and intercoms. Although ground to air communications came into limited use during WWI, it was not until WWII that radioing between planes became necessary to identify friend from foe and to coordinate attacks between crewmembers over the intercom. The earliest use I could find was from 1943.

"...which was divided into quadrants each designated by an hour of the clock, whether above or below or level with an aircraft...as a quick and handy way of pinpointing an attacker."

1 As was pointed out in a previous post, clock positions can be given in modern Polish, but there is no tradition of this in German or Russian. As Hevelius in Gdansk was also observing the moon at roughly the same time as Hooke, as well as Copernicus, Kepler, this makes sense.

2 Richard Lovell Edgeworth (1767) invented a semaphore system using arrows positioned at 45 degree angles. Possibly Chappe (1792) was the first (in French) to indicate position on his semaphore using clock position.