The naming convention is known as the NATO phonetic alphabet: https://en.wikipedia.org/wiki/NATO_phonetic_alphabet
Able was an air burst over Bikini (thus the name of the swimsuit).
Baker, the water burst, was the world's first atomic disaster; as a result of Baker, the third scheduled test Charlie was cancelled. My father died years later of colon cancer, perhaps not unrelated to contaminated air and water at the Eniwetok base afterwards.
FUBAR indeed.
Bit offtopic: As well as general use, a lot of thesed are used to classify Soviet/Russien submarines from a NATO point of use.
Even more off topic:This is quite interesting (to me at last) in that NATO has used prefix schemes for bombers, fighters etc. (for example Bear (bomber), Fishbed (fighter)) rather than their makers names. As far as I know, in WW2 the Germans always referred to RAF fighters by their RAF names.
One character variable names for temp or iterator values are everywhere in programming. But I've never ever encountered one spelled out as a full transcriptions of the NATO phonetic alphabet like alfa, bravo, charlie. Exception is alpha for probability/statistics.
Not all. Military definitely favors NATO, but there are other phonetic alphabets in use. In particular, at least in the US, fire/ems personnel (and sometimes also law enforcement) use alternatives. The one that goes Adam, Boy (or Baker), Charlie, David, Edward, Frank, ... is still widely used.
I've also known agencies to use a mix, like Adam, Baker, Charlie, Delta, ... (a law enforcement agency that I dispatched for back in the 1990's used this version).
Source: was a firefighter and 911 dispatcher in a previous life and still spend a lot of time monitoring fire/ems channels locally just to stay connected to that world.
I don't tend to use single-letter variable names outside of the standard `for(;;)` syntax, but if I did, I don't think I'd replace them in this way.