What does "schleterkadavski" mean? Is this a real word? It shows up on
scavenger hunt lists. Is this a joke?

This isn't an authorative answer, but is probably the best you will
get - all is quoted:

The word should actually be spelled: schlechterkadaver, which
translates as a carcass or corpse in a worse way, i.e., a rotten
carcass or corpse on a ski.

schlechter: worse, or in a worse way

Kadaver [kadavr]

(pl. -s, -) (m) carcass.

Ski: Norwegian, from Old Norse skdh, stick, snowshoe. See skei- in
Indo-European Roots

Now, as one might guess, such an object would be difficult to procure
and; therefore, it was always included on the list for advanced
scavenger hunts as a sine qua non nonpareil ne plus ultra.
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