Hapax : n.m. Word occuring only once in a given corpus (especially with old languages).

Each hapax is interessant because its translation can't always be sure, for lack of examples. In French, there are many hapax legomenon in Rabelais's or Proust's books. As for me, I choose the word "ptyx", that we can find in a Mallarmé's poem: "Sonnet allégorique de lui-même", or more simply "Sonnet en yx".

Difficulty : *****

Tested, logical.
Don't require any specific method, but you'll need to be very, very focused.
Good blackenage.