TheEnglishMaster:
Um ... I always thought it was 'deserts' as in 'what you deserve' and 'just' as in 'justice', but Jools' wobbling blancmange is, I confess, a far more tempting interpretation.
That is the original meaning of the phrase. "To get one's just desserts" is to receive exactly what one deserves. However the various alternate interpretations mentioend above are far more creative and could be used interestingly.
People often use a standard phrase with little or no awareness of its original meaning. For example the well known phrase "The exception proves the rule" which people take to mean that somehow finding an exception to a rule establishes its validity. Not at all. This uses an older meaning of "prove" still active in such uses as photographic proofs, "prove" = "test" So the exceptions to a rule test the rule, and help show its purposes and limits. Far more sensible, in my view.
Another using "proof" is the now common phrase "the proof is in the pudding" which makes no sense at all. This is a shorthand or corruption of "The proof of the pudding is in the eating" meaning that the only way to test the result is to try eating it, or m ore generally that it is only by trying to use something that one can be sure of its quality. Here again, "Prove" = "test".
There are a number of similar idioms and common phrases whose origins and original meaning are now obscure. Perhaps people would be interested in a separate thread about them?
-Don L.