"disease of the lungs characterized by progressive disintegration of pulmonary tissue" (usually synonymous with pulmonary tuberculosis, consumption), c. 1300, tisik, pthisic, tphisike, etc., from Late Latin phthisis "consumption," from Greek phthisis "wasting, consumption; perishing, decay; waxing," from phthiein "to decay, waste away," from PIE root *dhgwhei- "to perish, die away" (source also of Sanskrit ksitih "destruction," ksinati "perishes"). The restored classical spelling is from early 16c.

late 14c., tysyk "of or pertaining to a wasting disease, wasting the flesh," from Old French tisike, phtisique "consumptive" (11c.), from Latin phthisicus (Medieval Latin ptisicus, tisicus), from Greek phthisikos "consumptive," from phthisis "wasting, consumption" (see phthisis). It is attested earlier in English as a noun meaning "consumption, wasting disease of the lungs" (mid-14c.). Related: Phthisical.

The old pronunciation dropped the ph-, but this will probably recover its sound now that everyone can read. [Fowler]