Charles Matthews I think this may be a more general term, implying a bad-looking shape that is good in context.
John Fairbairn Charles is right that it's a general term. The rest of his definition also fits go praxis well, but I wonder if there is a subtle nuance of hesitation missing. I have tried, probably unsuccessfully, to cover this with my own rendering as "deliberate bad shape" (not necessarily good but certainly deliberate). I've never got round to looking this up in an etymological dictionary. It's not part of the general language, but I've assumed it comes from the adverb/adjective guzu-guzu. This has a range of meanings, but the most important are probably (1) of doing something sluggishly or hesitatingly and (2) fretting or complaining. The latter meaning is turned into a verb as guzuru. -mu is another way of creating verbs. I therefore am disposed to think that the idea behind guzumu is of doing something reluctantly (while complaining, of course!) only because you have to - because it is the best thing under the circumstances. I.e. good in context as Charles says, but not good per se.
Hu: May I explicitly suggest "reluctant shape", which was only implied above? I am only going by what John Fairbairn says above, but I suggest this as less cumbersome than "deliberate bad shape". It seems less stringent than "forced shape" and more understandable than "grudging shape" or "begrudging shape". I get a sense that the meaning is like "making the best of a bad job".
Jan: I translated my first definition from a page in French found via Google. It says it means absurd group and then goes on to say it's an good empty triangle (if my French still serves me correctly), but it makes sense that there's a more general meaning. By all means update this page after we've settled on a good word!
exswoo:Are you sure about the more general meaning? As I understand it, the guzumi term developed from the idea of a "dulled kosumi", or a "guzu-kosumi", which would mean that it can only apply to triangle shapes by definition...
John F. Though in practice an empty triangle is nearly always involved, the thinking behind the term goes beyond that - the point of the discussion. So, yes to the first question. As to dull kosumi, I don't accept that for one moment. It defies all the usual laws of word contractions in Japanese, and there is a ready-made model at hand with the same root: guzu. Thus we have guzuru and guzutsuku. So clearly guzu is a morpheme that enters into word formation without corruption, and since -mu is a standard bound morph forming new verbs, guzumu seems a perfectly regular construction.
I've never seen a rendering of the word with any character that implied kosumi. I have seen 愚集む and 愚図む. The former is clearly a jukujikun reading; the latter is maybe just ateji. But at least the former does show what a native Japanese feels lies behind the term: a stupid agglommeration of stones, overconcentration (in general). Folk etymologies are always suspect, but it's clearly jukujikun because otherwise it would be a bungo shimonidan verb and the renyokei would have to be guzume. Interestingly, guzu is used as a synonym for shuu (written with the 集 character) in the whole range of phrases guzu X moku, where X is a number, for the shapes that occur in nakade positions. On that basis we could conclude that the base meaning of guzu here is "clump", rather than the usual dull/slow dictionary definitions, and so the meaning of guzumu is simply "make a clump (overconcentrated) shape." But that still doesn't tell us where the nuance of its being unexpectedly good comes in. In fact, I'm not entirely sure that it does. If we assume that pros don't normally make bad shape and therefore the term is only ever used on the rare occasions that they do - because it works - the extra nuance is perhaps really in the context rather than in the term itself.
exswoo:*shrugs*. It was something that I was told by some Japanese Go players at a Go Salon. It seemed to make sense at the time since we have the popular Japanese tradition of conjugating two words make 1 short one. Actually, I was looking up the word in a Japanese dictionary (not an on-line one) and I was surprised to see a definition of the word listed, which goes "愚集み-A move in Go. While this move creates bad shape in relation to the other stones on the board, it becomes a good move from time to time."...which lends to your idea that it doesn't assume that the clumped shape is good or bad, just that it occurs and that it might be good according to the circumstances.
Bob Myers:My impression is that guzumi is a move, whereas empty triangles etc. are shapes. In other words, ten or fifty moves after a guzumi was played one would not look at the board and say ah, that's a guzumi shape. A purely academic distinction?
some guy in 2020: That's fun. A term that means "contextually-good bad shape" that only acquires the meaning of "contextually-good" contextually.