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]