Phrase

[Bongo_mfc]

Does anyone know where the phrase 'Tight as a gnats chuff' come from?  Who knows?

[Quizmonster]

It's just a variant on lots of other 'tight' comparisons such as 'tight as a duck's/camel's ar$e' and so on. You could probably make one up yourself here and now. How about 'tight as an earwig's butt'? 

[Xollob]

I don't know if the magazine invented the term, but I first encountered this phrase as a description of the Viz character Norbert Colon.

But as Quizmonster rightly points out, there's nothing clever about it. How about "tight as a wildebeest's sphincter"? See, anybody can do it.

[jno]

note however the optional extra clause with fish's/duck's ar$e - 'and that's watertight'.

[Quizmonster]

True, Jno, but in the case of the camel, one could add: "and that's wind-driven-sand-tight".

[Xollob]

...whilst with the wildebeest's sphincter, the usual add-on is "and that's Kalahari-duststorm-prooof".

[richship]

It goes back further than Viz - Albert Steptoe definitely used it during an episode of Steptoe & Son back in the 1970s.  (I don't think it was a 1960s episode though I could well be wrong)