Its adding wood pulp to ice to make the ice much much tougher. It was invented by the epitome boffin Geoffrey Pyke, who proposed they build an aircraft carrier of it! https://en.wikipedia.org/wiki/Project_Habakkuk
Anyway, I bring this up because friend of mine once decided to encase a small gift in the centre of a bucket full of pykrete.
The joke obviously would that the receiver would just have to wait for the thing to thaw because it was impossibly hard to break it. Then they discovered that somehow, the woodpulp in pykrete makes it extra insulating. On top of that, since it wasn't a flat slab but a roundish sphere (low surface area compared to volume), it didn't exactly absorb ambient heat quickly either. So that other friend apparently had to wait over a week before the thing was thawed enough break open.
Speaking of which, he's first cousin to Magnus Pyke - the guy usually remembered in the US (if at all) as the Professor shouting "science!" in Thomas Dolby's "she blinded me with science;" both on the song and in the video. In the UK he was, like his cousin Geoffrey, much better known for being the quintessential boffin, though Magnus self consciously played up this stereotype in his long television career. Sort of like Bill Nye on the outside but Carl Sagan on the inside.
Archetypal example would be someone like Sir Barnes Neville Wallis or someone more recognisable around these parts Tommy Flowers.
"Boffin is a British slang term for a scientist, engineer, or other person engaged in technical or scientific research and development. A "boffin" was generally viewed by the regular services as odd, quirky or peculiar, though quite bright and essential to helping in the war effort. The World War II conception of boffins as war-winning researchers lends the term a more positive connotation than related terms such as nerd, egghead, geek or spod."
To explain the modern use (which is very rare, outside of the tabloid news industry), a boffin is somewhat similar to geek, but it's used to describe knowledgeable people in academic / research fields, and occasionally in computing too. It's often said in a jokey way (I've never heard anyone describe themselves as a boffin), but it's not really an insult, there's a certain level of recognition that they know things that the person calling them a boffin does not.
But yeah, he was eponymic to Pykrete too :)
Lovely link, by the way, I did find Pykrete very interesting!
Just FYI, it's common to use "of" or "of a" after "epitome", for example "he was the epitome of a boffin".