"Guts pose" (ガッツポーズ) is the Japanese word for... something I don't think there's a universally-recognised phrase for in English. "Victory pose"? "Triumphant raising of fist or fists"? (JeKai notes that the fist-pump is a common English equivalent, in spirit if not in specific muscle groups activated.*)

I don't know if there's a specific linguistic term for "word that is made of loanwords, but is not itself a recognised lexical item in the language that those individual loanwords were loaned from", but that's what "guts pose" is. It was allegedly coined to celebrate the specific guts of one Guts ISHIMATSU, a pro boxer who defeated the then-WBC lightweight champion, Rodolpho GONZALEZ, on this day in 1974. A reporter dubbed Ishimatsu's subsequent arm-raising exuberance a "guts pose", and a word was born.

That's the most common story, anyway. Wikipedia notes that the Japanese term "guts pose" may actually come from 1960s bowling slang and/or an early-70s magazine called Guts Bowling. Still, it seems likely that Ishimatsu and his media presence helped popularise the term, and Guts Pose Day (ガッツポーズの日) itself is, of course, directly traceable to his bout with Gonzalez.

* JeKai actually has a lot of interesting stuff, as you'd expect from an open dictionary with a stated goal of depth rather than breadth. There's even an entry about the mysterious "PART ALTERNATION MARK" at Unicode 303D which we discussed in comments a few weeks ago. Apparently it's called 庵点, ioriten = "hermitage mark", because it looks like the sloping roof of a traditional iori hermit cottage.