What is the origin of the phrase eyes like pissholes in the snow?
The New Partridge Dictionary of Slang and Unconventional English (2008) says:
eyes like piss-holes in the snow noun deeply sunken or squinting eyes (whether naturally, or as a result of illness, or – most commonly – as a symptom of a hangover) UK, 1970
However, A Dictionary of Catch Phrases (Partridge, Beale, 1985) gives an earlier date:
eyes like piss-holes in the snow (usu. prec. by he has or he’s got). ‘One of the most graphic phrases that is applied to the aspect of someone suffering “the morning after the night before”: he’s got eyes like....’ (Brian W.Aldiss, 1978): since c. 1920.
A possible 1952 snippet is found in the Tennessee Folklore Society Bulletin (Volume 18, page 17):
816. Her eyes look like two p--— holes in the snow. (i.e. "eyes have a hollow look")
I saw him the other morning about five-thirty come creeping along through that fog about fifteen miles an hour, eyes like piss holes in the snow, drunker'n nine thousand dollars.
Another variation is "eyes like piss-holes in the sand" and the OED has this from around 1932:
c1932 D. Thomas Let. in Sel. Lett. (1966) 4 My eyes are two piss-holes in the sand.
The military attribution seems to be accurate. "Piss-hole in the snow" is a negative assessment of value (around World War II) much like "rat's ass". Note that "rat assed" also evolved to mean drunk.
Sometime between 1965 and 1970 the expression "eyes like pissholes in the snow" came to be used for hungover.
You can see its culmination in the movie "Get Carter" (1971) as an indication that the expression entered the mainstream.
I do not know for certain, but I would guess this is a British street-culture adaptation of a WWII expression that got picked up by writers and introduced to common media.
Also please note that the Dylan Thomas reference is not a Metaphorical use of "Pissholes int he Snow" as it only means bloodshot eyes from overwork and stress, not hangovers.