>>...Slang often derives stuff from its own stuff. Like "where are your reindeer?" being equivalent to "how's your Alzheimer" by the following line of reasoning: "izlapeti" - lose strength by evaporation (said of solutions, smells), i.e. go senile; therefore "laponac" - word usually used for people from Laponia, i.e. the Sami, now comes to mean a senile person. The reindeer question is just the next iteration.
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>>But the rhyming slang wins hands down as being cryptic to the uninitiated :).
>
>I presume Laponia is Lapland, in Finland, and the Sami are the Suami, ie Laplanders (or Laps).

Yep - I've found the Sami previously, tracing some (language?) stuff on Wikipedia, but these were in Norway. Multiple spellings are to be expected - when they have multiple names already.

>Where is the rhyming slang BTW?

On your side - seeing how the above could be complicated to explain, I remembered your descriptions of the rhyming slang, and it won "for breasts" (i.e. one breast depth ahead on the finish line).

>There's an old Brit saying for crazy, only used by us who can remember pre-decimal currency - "Tuppence-ha'penny short of a shilling" (which nowadays is equiv. to c. 4p, which doesn't have the same ring to it :-)

We had a "not all of his sheep on count", which initially meant a sad or just downbeat person, i.e. as happy as a shepherd who lost a sheep. That somehow got mixed up with "missing a plank" (which later expanded into "missing a whole door", "missing a lumberyard") i.e. there's a hole and there's a draft through his head... so in the end you get "not all of his on count" (sheep or planks got omitted) to simply mean the guy is understaffed in the brains department.