This is a modern version of a Latin word, 'miscellaneus', meaning composed of different or mixed elements. The Latin word itself comes from the verb 'miscere', to mix. The modern form of the word goes back to the 17th century.
The word 'miscellaneous' is most often found in lists and filing systems - anything that doesn't fit into an obvious category can be filed under 'miscellaneous', which is why the miscellaneous list or file is often the most interesting one.
A miscellany, again a word that you first find in the 17th century, originally meant a collection of pieces of writing by different authors (which we would more usually call an anthology now). Now it is more often used to suggest an odd collection of pieces, unusual facts etc. The most famous example is probably Schott's Miscellany, which really has a bit of everything.- - useful jpractical information, bits of folklore, nursery rhymes, odd historical facts and much more.