'A wasting away' is a noun phrase formed from the phrasal verb 'waste away'. Click HERE to read about its meaning.
Student or Learner
phthisis \THAHY-sis\ , noun:
1. A wasting away.
2. Pulmonary tuberculosis; consumption.
At last Sister Hyacinthe began to speak of the immediate and complete cures ofphthisis, and this was the triumph, the healing of that terrible disease which ravages humanity…
-- Robert Hugh Benson, Lourdes
Apoplexy is no longer to be feared, but phthisis is there. Social phthisis is called misery.
-- Victor Hugo, Les Misérables
" A wasting away" What does it mean ?
"away" is a noun, isn't it ?
The chances that you will need to know the English word "phthisis" and use it in a conversation with a native speaker are slim to none.
I don't know the Lourdes book, but its name, and the character 'Sister Hyacinthe', suggest that it may be partly or wholly set in France, and the other example is French (although English pronunciation would make the stress on 'Victor Hugo' unrecognizable to his compatriots). It's a feature of the French that they are more at ease with academic words: for example, the word 'carious' exists in English, but is used chiefly by dentists. Ordinary people say 'rotten' or 'decayed'. But a French speaker uses carié in normal conversation.