borrowed from New Latin nosologia, from Greek nósos "disease" (of obscure origin) + -o--o- + New Latin -logia-logy
Note: Attic Greek nósos is paralleled by Homeric and Ionic noûsos, suggesting an earlier *noswos, though the Ionic form (chiefly Herodotus) may be taken from Homer; then the possibility arises that noûsos in Homer should have been read *nóssos, in which case *noswos is out of the question. In any event, neither form leads to a plausible etymology.