late 14c., tysyk "of or pertaining to a wasting disease," from Old French tisike, phtisique "consumptive" (11c.), from Vulgar Latin *phthisicus, from Greek phthisikos "consumptive," from phthisis "wasting, consumption" (see phthisis). Earlier in English as a noun meaning "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]